MIAMI (AP) _ Touched by the plight of a 7-year-old boy who needs a liver transplant, sympathizers from a millionaire industrialist to fast-food restaurant workers followed President Reagan's lead with more than $400,000 in donations Friday.

At least three businesses called to pledge $5,000 each, quickly offsetting the more than $4,000 raised by Ronnie DeSillers' classmates but stolen Thursday from his school in Fort Lauderdale.

Volunteers said a $200,000 gift from Victor Posner, one of the nation's highest-paid corporate executives, helped the total pledged exceed $400,000 even though they hadn't been able to come up with a complete tally. They said they will continue raising funds, with $1 million the goal, to ensure that all medical costs are covered.

''The calls are coming in so fast we haven't had time to add up today's figure,'' said a man who identified himself only as Tom, answering the telephone at a hot line set up to take pledges for a bank account at Coral Gables Federal Savings and Loan.

The stolen money was part of $89,000 raised to that point for Ronnie, who needs $162,000 to get onto a waiting list for a transplant at Pittsburgh's Children's Hospital. Final costs could be $300,000 to $500,000, an expense that his mother, a single parent, can't meet.

Maria DeSillers, 31, lost her medical insurance 4 1/2 years ago when she was unable to make payments because of an extended absence from work to be with her only child during one of his hospital stays. Ronnie was born with a condition that caused cirrhosis of the liver.

With the check from Posner and other money coming in, the boy could be moved to Pittsburgh early next week, said George Rodriguez, spokesman for Miami Children's Hospital.

Lynn McMahon, a spokeswoman for the Pittsburgh hospital, said Friday night that surgeons have to examine the potential candidate to determine whether a liver transplant is necessary before the person can be put on a waiting list. She was not able to confirm whether Ronnie has had such an evaluation.

The hospital would not say how many children would be on the waiting list ahead of Ronnie, but that about 90 are on the list at any one time. However, McMahon said that when a donor liver becomes available, it goes to the most compatible candidate.

Ronnie, who is at Miami Children's Hospital, was cheered by telephone calls Wednesday from Vice President George Bush and President Reagan, who promised he would send a donation.

But the boy is growing weaker by the day, Ms. DeSillers said, and doctors have said he will die in six to 18 months unless he gets a new liver.

''Ronnie has not eaten anything the last two days. He is getting a lot weaker. I want to move him in the next few days,'' she said Friday.

''I'm scared to death, but I'm very, very happy and touched to see how the community is working together,'' said Ms. DeSillers, who works in advertising.

After news of Reagan's call was followed by the school theft, fund-raising accelerated and local music groups and radio stations began holding ''Ronnie- a-thons.''

Thursday night, more than 1,300 people paid $10 each to attend a benefit concert at a Miami Beach nightclub, and thousands more in pledges were received.

In four hours Friday morning, Hollywood pop music station WHYI attracted about $30,000 in pledges, said morning disc jockey Sonny Fox, adding that Pegasus Air Ambulance promised to provide a Learjet and medical staff to move Ronnie to Pittsburgh.

Fox began his show by playing recorded interviews with Ronnie and his mother.

''Just hearing his voice, this cute, intelligent little boy talking about his disease, did it,'' Fox said. ''Once, he asked his mother: 'You're not crying because I'm going to die, are you, Mommy?'''

Three businesses quickly called to pledge $5,000 each and employees of six Castle Burger fast-food restaurants took to the streets in their uniforms to collect funds.

The radio station also auctioned off $150 to $300 value weekend vacation packages donated by beach hotels.

Some donors called directly to Ronnie's school, St. Anthony's Catholic School, in Fort Lauderdale.

''I've gotten calls from Chattanooga, Tenn., Minnesota, Detroit and Frankfort, Ky.,'' said school principal Elaine Patterson.

Radio stations planned to accept pledges all weekend and members of the Metro-Dade police SWAT team were to solicit contributions. Miami City Commissioner Joe Carollo proposed that the city collect money and ask promoters for donations at next month's Genesis rock concert, expected to draw 70,000 people to the Orange Bowl.

Friday's outpouring of pledges ''has been wonderful for these children who were shocked'' after thieves broke into the school early Thursday and stole more than $4,000 students had raised, Ms. Patterson said.

The school's eighth-grade class recently voted to give up $2,000 it raised for a trip to Disney World and banked that money for Ronnie.