I'm so sorry you lost your mum. That's a really tough thing, especially at such a young age.
There are so many red flags in your opening post. I'll just go through them:
We are exclusive but he doesn't want the commitment of a relationship Yet you've been seeing him for a year - he can't even accept it's a relationship? What a waste of time he is.
I stay at his place once/twice a week Does he ever trouble himself to come to yours? Is all of the effort down to you? If you couldn't get there for whatever reason, would he come to you? Pay for a taxi for you? Come to get you?
My birthday rolls around and he tells me he's overspent this month and therefore won't be getting me anything (not even a card) He's absolutely horrible - he's not even a friend to you, never mind a boyfriend - and worse still I got him some lovely gifts and a card and we spent a nice evening together in lockdown for his birthday. He's a selfish user.
Despite the fact that over the few weeks leading up to it, he'd been out multiple times with other people - not me. He went out with 2 guys from work and a female from work too without asking me once. He doesn't even see you as his friend, never mind his girlfriend. He spent all his money out with other people and wouldn't even buy you a birthday card.
I even said the weeks before, I just want to have a special night with him as it's my first birthday without my mum (she passed away) - I didn't expect anything big. Just to feel special. He couldn't do that. In his mind, he's the special one, not you. Again, he's selfish, thoughtless and a user.
The night of my birthday comes and I got a 15 minute call at 11:30pm after he'd been at work and gaming You can see exactly where you are on his list of priorities.
I may come across as expecting too much since we aren't official You really aren't expecting too much. You have to value yourself more.
I don't understand how you're not in a relationship with him but you're exclusive. It wouldn't surprise me if he wanted you to be exclusive (as in not seeing anyone) but doesn't apply the same rule to himself.
Think what your mum would advise you to do. Would she want you to be with someone who values you so little? Who's so utterly selfish and thoughtless?
I have a daughter around your age and the thought of someone not valuing her would just break my heart. Think how much your mum loves you - that's what you deserve in a boyfriend. Think how your good friends treat you - that's what you need in a boyfriend.
This man is absolutely awful and I think you should think about why you're putting up with so much shit from him. Did your mum have a good relationship with your dad? Can you think of a couple you know who have a really good relationship? If you can, next time you have a boyfriend, measure him against the guy in the couple where you know it's a healthy loving relationship.