I think it's a shame that your mother told you and this man these things without being certain, as it's clearly had a big impact on two people's lives.
That aside I would try to avoid striking up a close relationship or friendship with this man at the moment. After what you've both been through your emotions will be all over the place and you'll both be feeling differently about what you could've had. I would be polite and friendly, but remember your only connection to this man is that your mum had a relationship some 30 years ago. You don't owe him anything and I just think it would be beneficial to you all to have a bit of distance between you for a while and then.see how things are when emotions have settled down.
There's nothing wrong with having a friendship, but be careful that neither of you confuses this to be something it's not
I too would advise caution. You wont be in any frame of mind to make good decisions.
I think back to when I lost my dad, I was 21. Someone I sort of knew had just lost his daughter in a freak accident. He was 42 and we ended up having a relationship. His long term girlfriend was not a happy bunny when she found out (I didn't know about her) and it all ended in a mess. I can laugh now but at the time it was awful. Just a cautionary tale about looking for fartherly figures when we are perhaps not in the best frame of mind to think rationally.
At the moment you're both feeling the same sense of loss that things weren't as you expected them to be. He's not your biological dad but was obviously willing and eager to take on a relationship with you in that capacity and you've clearly bonded.
What's changed? A piece of paper doesn't negate what you both were building. If it was me I'd give it a go and see what happens in the future.
The fact that you are asking on here makes me feel that you are not 100% sure and therefore you want to err on the side of caution but having had all these disappointments in the past you so want things to be right too.
Take things slowly, at a comfortable pace. If this man cares for you then he will appreciate your past is affecting you and accept things on your terms. You could both be a wonderful help to one another.
I feel for you I really do. About 8 years ago a pregnant teenage girl turned up on my mums doorstep looking for my dad (who had left some 15 years ago) it turned out my dad was her dad too after he had an affair with her mum and all that time she had been living in the next street. I was devastated, I thought I was his only daughter and that had meant something to me. We never asked for DNA, she looks a lot like me. I didn't want to hi-jack your thread, just let you know that I know how you feel about families being the most complicated things in the world.
Moving away from the relationship issue, but back to the fact that you are unable to find your real dad, could you perhaps seek some counselling? Someone who could help you put your thoughts and feelings into order?
I can not give you any advice but just to let you know my dad went through the same thing and only just last year found out. So confusing as he has different surname but for me my grandad will always be my granddad regardless