Hi and welcome, Sally, I’m so sorry you’ve been hurt like this. No one except your boyfriend can tell you why he’s made this decision. There could be any number of reasons - he may have felt this way for a while but didn’t know how to tell you, he may be finding the balancing act between his exes, his children and you too hard to cope with. It may be that he feels he’s a burden on you and doesn’t want to continue doing this to you, it may be he feels you’re better off without him. Depression can also make you feel numb and this may mean he feels his feelings for you are suppressed - and this potentially could happen overnight.
I can’t tell you whether you should unblock him or not but I would say it’s early days to give up on him. He may just need you to gently tell him that you won’t put any pressure on him but that you’re there for him if he needs you. Let him know you’re not going anywhere right now and leave it in his court. Ultimately, it may not change anything but he may appreciate having the space to deal with whatever he’s going through