Hi OP. I get that you are hurting about this, it must be so hard. I know it feels like a long distance - and it is - I'm from the north and moved down south, leaving my mum. I felt a lot of guilt about it, but it was ultimately what I wanted to do.
I do miss my family, a lot sometimes, but I've never been happier. And we are actually still really close.
I hope that it will be the same for your son. Brighton is a great place and he will have adventures there. He's 22 and he should be going out and exploring new places. All the most interesting, well rounded people I know have lived in different places and had a variety of life experience - he really needs to get that.
He probably will make some mistakes and struggle etc., but that is part of being in your 20's. He will muddle through it and become more resilient.
This is what it's supposed to be like when you have adult children - don't clip his wings.
Will you be able to visit? It's not a bad train journey down South really! It might give opportunities for holidays for you and spending weekends together etc. I see my parents fairly infrequently, but when we do see each other, we have a good amount of time together and really enjoy the closeness.
I hope it all goes well for you both - I'm sure you will get there and he will get some great life experience!