All the advice below is great, especially the ones noting that for boys, the sensation of sitting to poop is a new experience - girls must sit to do anything at all on the toilet. Learning to pee and learning to poop in the toilet are two very different things for boys, so think of it as two different types of potty training, not one single effort.
I would just add that you should know some kids think of pooping as somehow "losing" part of their bodies, so they resist it. Try the tactic of waiting to flush his poop after he has left the bathroom so he doesn't see it go down the drain, so to speak. Also, distraction is key -- be sure you have tempting new books he can see ONLY when sitting down to poop: "These are special books for when you sit on the potty." Ensure they're ones he has never seen and are ones that will definitely keep his attention for more than a few seconds. This can help a child both sit long enough, and relax long enough, to forget why he's there, and then his body might relax enough to let the poop go.
Think of this, too: It is actually a good sign that he "asks for a diaper to poop in" because it at least means he is aware when his body is ready to poop. If he were having accidents all over the place that would be worse.
Do not stress out over the "age where I should get concerned." There is NO magic age limit by which a child should be perfectly potty trained. Search Mamapedia and you will find parents of kids of many ages dealing with many different potty issues including this exact one -- peeing but not yet pooping, and at ages older than your child's age. If you stress about this, or force him, he will pick up on your stress, which will make him tense and it will only take longer for him to make this transition.