Hi.
I'll try to keep this short.(ish)
2y 11month old DS. Still in nappies. Fussy eater. Although can eat a LOT of what he does like. Lots of cereal and milk, banana milkshake (real, homemade), bread, almonds (sliced), pasta, cheese, pate, fruit juice, baby smoothies, sausages
, fishcakes 
, chips 

.
He has for a long time now not been very frequently opening his bowels. The first time this came to light was when I tried to potty train him by taking his nappy off last summer. He had been showing lots of signs of knowing when he needed to go and interest in visiting the loo / washing his hands.
Anyway, after a couple of days he ended up with compacted constipation and I had to give him laxatives. The size and state of the poo suggested it had been in there a while, so I think it must have been brewing before the nappies came off.
Ever since then he goes just a couple of times a week. Occasionally less often. It will be clear that a poo will be brewing for a couple of days before it actually materialises. Eventually he can't stop it coming and he'll fuss and cry, not wanting to soil his nappy but not wanting the potty either. So I'll hug him and keep him on the potty while he does the business.
I had assumed the constipation was down to his poor diet. But now I'm thinking that actually he might just be holding on. He drinks plenty of water / squash / juice / milk and when the poo does arrive, it's usually very soft.
As of today, he's not had a poo since Thursday before last (10 days ago) and his tummy is huge. He's been really fussy the last couple of days, like you'd expect a poorly toddler to act, saying he's hungry but not eating much.
I'm going to take him to the GPs in the morning. But I'm not sure what it is I need to ask for. It's a long term fix I need. I just don't know what I'm supposed to do. Ride it out? Keep the nappies on? Take them off?
Does anyone else have any experience of anything like this and know who/what I should be asking for? I need a referral to an expert - but what sort, psychologist? paediatrician? poo consultant?!
He was born 12 weeks prem, although at this age I don't think that's relevant any more.
Sorry, that wasn't short!! Thank you for reading.