Not asking friend to be groomsman

It depends on the person. He may not even care.

My FH hasn't asked his groomsmen last July (way too early in my opinion) and one of his close friend's wife told me (while at another mutual friends wedding) that he had made comments about FH not asking him. He would have been upset if FH hasn't asked him.

You FH likely knows is friend best. How does he think he will react?

Luckily, weddings aren't tit for tat and you aren't obligated to ask someone just because you stood in their wedding.

He is not obliged to ask someone just because he was in their wedding party. If that were true all weddings would have the same people involved. Everyone has different circles of friends and relatives with different degrees of closeness.