Hi Damien
I really wouldn't worry about it (as long as there's no sign of an illness, of course). But bouts of picky eating is normal for puppies. The thing is, they grow in spurts. And during each growth spurt, their little bodies need all the energy they can get - so the pup has a big appetite to match. In between spurts though, the pup simply doesn't need to much energy. So he isn't very hungry, and he eats less. It's all perfectly normal, and more than that, it's
healthy 
So you can pretty much expect it to go on for a couple of weeks or so. Then the next growth spurt will start, and he'll dive right back into his food. When it stops, he'll eat less again. And so on. The big growth spurts are usually mostly done by around the 4-4.5 month mark though, and things usually even out from there.
Anyway, as long as he is healthy and bright and playing well, and is getting a good quality food, then there's really no cause for concern. Really, it is only if there were some sign of illness that you'd have cause to worry.
