So he's got a job now if he can pay the mortgage?
If the ownership & mortgage are joint you need to contact the mortgage lender yourself and find out what the position is. I cannot believe he can repeatedly miss payments without them being able to start repossession proceedings. [What you say implies he could go on paying just 1 month in 3 without them doing anything - that just is not on!] Once you know whether there are mortgage arrears, how much they are & what the lender intends to do about them you will be better placed to decide what to do yourself.
If you paid the mortgage for 2 years & he has paid it for 3 (assuming he has!) then on the face of it any equity should be split 40% to you & 60% to him, but only after the deposit paid by you parents had been deducted first. That too should come to you to pass on to them.
If he refuses to agree a sale you can apply to Court for an order for sale. You need a solicitor to advise you on whether this is likely to be granted, & on the likely split of equity.
Before spending money on legal costs, you should also get a realistic sale valuation as well as clarifying the mortgage situation so that you know how much equity there is.