A jury in Alexandria awarded $115,000 in compensatory and punitive damages yesterday to a 6-year-old Arlington boy and his parents for the short-term illness he suffered after being fed a defective infant formula.

The six-member jury rejected claims by the parents that their son also suffered long-term disabilities from the formula called Neo-Mull-Soy. It was withdrawn from the market in 1979 after it was found to contain dangerously low levels of salt.

The verdict in U.S. District Court concluded a $25 million suit against the formula manufacturers, Syntex Laboratories Inc., brought by Lynne and Larry Pilot. The Pilots, who helped organize a successful national campaign in l980 for stricter federal guidelines on infant formula testing, had accused Syntex, based in Palo Alto, Calif., of negligence and violations of the Virginia Consumer Protection Act by marketing the soy-based formula.

"I'm grossly disappointed in the jury's reaction to all this," Larry Pilot said yesterday after the verdict as his wife cried silently. During the nine-day trial they testified that their son Bradley, now 6, lost weight and at one point began to sleep with his eyes half open while being fed the formula in 1979.

The Pilots were awarded $5,000 in compensatory damages and their son $110,000 in compensatory and punitive damages for his illness in l979, which required hospitalization. The jury awarded nothing for the child's subsequent speech and memory problems, motor development problems and learning disabilities.

"The verdict is consistent with our long-held statement that we are willing to compensate any short-term illness caused by the formula," said Syntex spokeswoman Michele Klaich. "But we resist claims for damages related to long-term ill effects that are said to be due to . . . the formula.

"We have seen no persuasive evidence that Neo-Mull-Soy caused long-term injuries," she said.

Syntex has been the target of more than 100 suits across the country in which parents have claimed their children were harmed by Neo-Mull-Soy. The formula was recalled from market shelves by Syntex after it was found to have low levels of salt, a necessary ingredient for healthy development in infants.

Last month a Chicago jury awarded $27 million in punitive and compensatory damages to two children whose parents claimed the formula had caused them brain damage. Syntex has said it will appeal the award.

Lynne Pilot, a lawyer, was cofounder of a national organization of parents whose children had been affected by the product. The group, called Formula, successfully lobbied in 1980 for a law to enforce federally monitored testing of infant formulas. Larry Pilot was a Food and Drug Administration lawyer for 10 years and is a specialist in product liability law.

Judge James C. Cacheris took under advisement a motion by Syntex lawyers to dismiss the $5,000 award to the Pilots on the ground that the statute of limitations had run out when the parents filed suit last summer. Pet Inc., which had developed the formula, was dismissed as a defendant at the outset of the trial.