What is on the mortgage is not the issue. It is what provision has been made for the deeds. If you are joint tenants then you both share the whole value equally (split 50/50 if sold and you go your separate ways).If tenants in common you each own a specified share. Usually 50/50 unless one partner has a large contribution to protect. It doesn't sound like a TIC arrangement was put in place to provide for unequal shares.
He may do the right thing. If communication is good you could broach with him entering into a TIC deed asap so this is all in place before the sale. If he refuses or obfuscates you know you will have a fight coming up.
Prima facie he is entitled to 50%. That isn't necessarily the end of the road as there is a difference between legal ownership (how the deeds are held) and beneficial ownership. Beneficial ownership is where one party holds a legal share on behalf of another person - "on trust". It is possible for the person owed the beneficial sum/share to succeed in claiming it. You will need evidence of what each of you put in and if possible any evidence of agreement between you (letter, email, text etc) that your extra contribution is considered an extra share.
Best to get legal advice. If your evidence is clear and compelling he may back down quite quickly rather than have you start court proceedings.
This is why you see on threads posters advising an OP that if her new boyfriend moves into her home he should not make a contribution towards the mortgage or significant home improvement as he could stake a claim on the equity. This would be on the beneficial/trust basis. A friend of mine had her ex assert a claim on her house on the basis he had done a complete require of the electrics on the understanding that he was adding to the equity and would benefit. (She settled with him by paying him the trade value of the work.)
So there is that route. BUT if he fights dirty it could be legally expensive and stressful.
I hope you work something out that protects your money. Sorry you are having a tough time.
BTW I'm a solicitor but not a property one!