I gave 3/4 table spoon of children's Benadryl (diphenhydramine) to 5 year, 9 month old son. Will he be ok?

Probably. By the time you read this, if he is okay he likely will be. The effects are brief. It can cause drowsiness, but you would have seen this already. Always follow the directions on the bottle.
OK. If he happens to be about 65-80 lbs, that would be his recommended dose. If under, still not too far off as to be a problem. Watch dose instructions and "t" vs "t" (that would be teaspoon vs tablespoon, in case confused).