Pooping on the potty

My son who is 4.5 refuses to poo on the potty.  He was potty trained for pee pee at 18 months and slept without a pull-up by 24 months.  That was a breeze!  
When it comes to number 2 he wants a diaper.  I have tried him in the bathroom and I cut a hole in the diaper and he did sit on the potty once and was so excited, but will not do it again!  He can hold going to bathroom for days, which I think is not healthy.  He is also a bit constipated, so I do not want him to hold his poo.
When we talk about it, he says he is scared.  I have really tried everything.
I know eventually he will sit on the potty, but is there anything I can do to speed that up?
Thank you!

[lil_sweet_thang]

try having him sit backwords on the pot thats what we had to do with my nephew u can give him a book to look at and that way he will think of the potty as a good place not scarry and having him straddle the seat he wont feel like he is going to fall in

[lil_sweet_thang]

try having him sit backwords on the pot thats what we had to do with my nephew u can give him a book to look at and that way he will think of the potty as a good place not scarry and having him straddle the seat he wont feel like he is going to fall in