My DS has had chronic constipation for as long as I can remember - he's now 20 months.
He was taking lactulose, but with little success and about 5 months ago was prescribed movicol which he now takes daily - one sachet per day in his drink each morning. He now has daily, regular bowel movements and doesn't spend all day straining like he used to. I used to have to manually help him to pass a poo when things got really bad but now I don't. The GP we now have is brilliant, really sympathetic and doesn't try to make out that we are giving our so with a really poor diet - as we have been dealing with this since DS was weaning his diet consists of fruit, veg, brown bread, rice etc - he has never eaten anything other than wholesome foods due to the issue.
The movicol isn't and can't be forever and the plan is to start reducing the dosage from next month - he currently takes one sachet in his first drink of the day, next month we will be reducing it to every other day and reviewing progress.
We have never really got a diagnosis for DS other than being told that this is incredibly common, some babies just have a slow bowel and that it does become a vicious circle in that they start to withold and then it becomes even more painful and then they continue to withold etc. Movicol is only prescribed to babies under 2 in exceptional circumstances and it's not the ideal solution as you are depending on medicine to get DS to do something which should be straightforward. From my reading there are issues in prolonged usage such as control issues etc so we are hoping to be off the stuff before we start potty training.
If the reason for the ongoing issue is witholding and the vicious circle this becomes then it may be worth trying this as it may stop the psychological issue that may be the cause of the problem so that you are able to 'reset' the behaviour as we are hoping to achieve.
I hope this helps, I really sympathise with you, if your situation is anything like ours your life can become one long battle of the bottom!
Maybe your best starting point is trying to find a GP who sympathises and takes your concerns seriously. It feels like it's been the key to us getting to this point and feeling positive about being able to help DS move on from here.
Good luck!