People ask me this every year before my kids' parties. I usually do give suggestions to anyone who asks me directly. I'll say something like, he would love a new book or he's really into legos right now, or he has been really into art projects lately. That way, they have the chance to get something that he'll like, but within their budget.
The way I see it, unless you specifically say "no gifts," people are not going to come to a birthday party empty handed. So if they are asking, you might as well give them ideas.
When my daughter turned one, we really did not need anything for her and I knew she'd be getting gifts from family. I tried telling the friends that came to her party not to bring gifts; some did, some didn't.