Gut problems

My son is 4 years old with ASD and has just started school. Since he was a little baby he has always struggled to poo. He still experiences real difficulty and is still in nappies. The school are very understanding and well-trained, and for now, happy to change him throughout the day. But, I'm aware we need a longer term plan to get him out of nappies.

Problem is, I just can't see how. He rarely passes anything solid [apologies, about to get graphic!], most of the time just managing to pass a tiny 'skid'. If you put a nappy on, within 15 mins he will have soiled it again. Occasionally, he will have a day or so when he doesn't stop - obviously his body trying to clear out. To be honest, the whole issue of nappies is not my immediate concern, I'm far more interested in what I can do to help him. 

He now shows visible signs of being in discomfort, usually clutching his tummy, writhing about and becoming very irratable. His diet is inevitably poor too. I'm sure part of this is to do with routines that have been developed, but his diet predominantly consists of toast, yoghurts and bananas. We have tried to get him to eat all sorts of different foods, but he simply refuses. His older sister eats a huge variety of good foods and has a well balanced diet, but our son barely displays any appetite at all.

Our Health Visitor prescribed Movicol, which hasn't made much difference; they recently upped the dose without any noticeable signs of improvement. We've also been given something similar to Senokot - a laxative syrup. I'm concerned about the amount of laxatives we are giving him - instinctively it feels wrong to be filling him up with all sorts of these different remedies.

Would really appreciate any thoughts or suggestions, as we really want to help him go to the loo more comfortably and to enjoy a more varied diet.

Thanks