If you've been trying for 5 months I think you may have started too young. I know you may not want to but it may be better to leave it for a couple of months and try again. I bet before the 2 months are up he'll start showing an interest himself.
If you are determined to carry on have you tried letting him read a book while he's on the potty or fetching it into his play area and letting him watch TV. He may be so engrossed in what he's watching/reading that when he needs to poo he'll automatically do it rather than conscentrating on holding it in for his nappy.
Hope this helps

Yes, I hadn't noticed that you said you've spent five months trying to train him. When he's really ready, you'll be able to train him in about a week - honestly! He needs to know when he's ABOUT TO do a wee or poo, and to be able to communicate this to you (in words or gestures) and to be able to manage his own clothes (easy clothes like tracksuit bottoms are ideal). If he's not really ready, you're just giving yourself and him a lot of hassle by trying. Good luck!