Mine seems to have different moods for feeding. Sometimes she rears up and strikes almost immediately. Sometimes she shies away from it and I'll have to set the thing down on the ground and just leave and check back in a few minutes.
Feed #6, I had to set it down and only after about 10 seconds she slithered up all casual and slow... then ate the thing like a hotdog. No strike, no constriction, just opened her mouth calm as you please and started chowing down. Feed #7, she struck like a bullet while I was still lowering the thing into the tub.
I'd say they just feel like different things on different days
edit: Oh, and sometimes I have to make the poor little f/t 'dance' or else she won't strike, but other times I have to lay them down or else she seems agitated by the whole feeding situation. So I wouldn't worry to much about your baby only wanting f/t as a health issue. Maybe he's just mellowing out or realizing that f/t are easier to eat or something.