Are you saying that when you put puffs on his tray he doesn't pick one up and try to put it in his mouth. He shouldn't be self-feeding. He's not co-ordinated enough yet. But he should be picking up objects with a pincer (thumb and finger) movement at this age. The next step is to try to put it in his mouth.
If not, I would stop doing it for him, so that he gets the idea he can do it.
I wouldn't be concerned at this age since he's up to date with other milestones.