If your son is the one invited, he can buy gifts or not as he sees fit.
If I receive announcements, I send a gift when I want to, and don't when I don't want to. So many of them seem like money grabs. I haven't talked to you in 6 months and your kid in 4 years, and I get a graduation announcement? They are lucky to get a card wishing him/her well. If we are friends but not close, I send a small gift, like $10-20 in iTunes gift cards. If it is someone truly important in my life, I know them well enough to pick a meaningful and personal gift, and I'd spend more ore less depending on the relationship.
But again, that would be is I was invited or the announcement was addressed to me. My kiddo would be on his own. If he attends birthday parties of friends, do you buy gifts for that?