HAUPPAUGE, N.Y. (AP) _ A state appeals court has blocked a judge's decision to award custody of a 13-year-old boy to his homosexual father.

Legal experts said Acting Supreme Court Justice Morton Willen's award of custody to a homosexual parent was the first of its kind in the state. In New York, the Supreme Court is a trial-level court.

The court's Appellate division stayed Willen's decision Wednesday, asking the judge to consider new facts in the case.

The attorney for the mother, Arnold B. Firestone, said the boy, whom he identified as ''Brian,'' has tried to commit suicide several times and is hospitalized in a psychiatric institution for adolescents on Long Island.

Willen's 14-page decision, released Tuesday, does not name any of the parties involved, and identifies the boy by the intial B.

Willen rejected arguments that the father's homosexual life would adversely affect Brian, that Brian wanted to stay with his mother and expressed ''embarrassment and discomfort with the fact of his father's homosexuality.''

''The record indicates that B. fared far better with his father than with his mother,'' Willen wrote, noting that the father's eight-year relationship with his live-in lover is apparently stable.

''The court finds no evidence of any present or potential harm upon which to make the father's homosexuality a consideration in this custody dispute,'' Willen wrote.

The decision was issued after more than eight months of hearings, during which school officials, psychiatrists and other doctors testified that Brian needed strong parental control.

''It is not a question of this being a gay-rights case,'' said the father's attorney, Bernard Callan. ''It is a custody case. The only question is what is best for the boy.''

According to testimony, Brian's mother is chronically ill and has been hospitalized 80 times in the last 10 years. At those times, Brian and his brother and sister, who are 9 and 10 years old, often stayed with her parents.

Thomas B. Stoddard, executive director of the Lambda Legal Defense and Education Fund, a gay rights group, said he knew of no other such case in New York.

The dispute began when the mother said she was moving to Florida for her health and to be closer to her parents, who planned to retire there.

The father's attorney said his client sought custody only for the eldest child because the other two children are not having the same problems.