The boyfriend, who is over 25, won't tell me when he's angry. I didn't even know he was mad about something until he snapped at me over it a week later face to face and even cursed some instead of telling me sooner.

I'm not sure how to handle this or if I even should. I've told him to communicate with me before, but he still won't trust me enough to do it.

Have you sat down with him and told him that you are concerned about the lack of communication? You aren't a mind-reader and need for him to talk to you when something upsets him. Cooling off time is acceptable, but bottling up negative emotions until he explodes is not.

If you have tried talking to him and he still keeps doing the same thing, then you have to decide if his anger management issues are something you want to put up with. I, personally, wouldn't put up with it. I would walk away and let him figure out his problem is his own.

The thought might arise to give him an ultimatum, but you have to be prepared to carry through with the threat. Also, some people react negatively to ultimatums. Another problem with ultimatums is if the person does try to change it isn't because he sees a need to improve, but out of fear of losing someone. Most changes of that type only last as long as the person is afraid. A relationship based on fear from any source (his temper or losing you) is not going to be healthy.

Just as Cat1864 pointed out, first thing he needs to understand is, you cannot read minds.. and neither can he. So, you have talked to him about this before.. then, you ought to set a rule that if there is any problem or misunderstanding, it needs to be discussed on the same day. Or at least the next day. Tell him that, if he is going to bottle it and explode later, you are not buying it.
If he does it the next time after telling him, do not listen to a word, just leave from that place. Once or twice when he realizes that you are not going to intake his anger from the past, he will surely register in his mind to open up the henceforth. If I were in your position, it would really irritating if my partner digs and brings up things which happened two weeks ago, which I would have probably forgotten.

I can relate to this because it happened to me all the time. My girlfriend would go to a party or talk to a guy friend or something and I would be like fine its okay. Then I would for some reason get angry about it. I would curse her out for no reason what so ever. I felt like a **** after. But what helped me were 2 things. 1 after this happened for maybe a couple months she broke up with me. She did this because it helped me to realize how dum I was acting. During our break up stage I was depressed and sad and basically it hit me that I had to change how I viewed things. She took me back a little while after. Im not saying do that but I don't know it worked for me. Another thing that I personally do when I'm angry or about to blow up is GO TO THE GYM! My therapist recommended it to me and it has helped me ever since. My girlfriend would literally know if I was talking different or anything different and would say go workout. And I did and I would release all my anger and stress, I then after wards realized I was getting mad for no reason and calmed down. This is how we handled it and maybe it will help.

We all get mad.
Doesn't really serve us, unless we know why we have anger. How to turn it around. Understand why.

You both have to be able to talk & resolve. Listen. Talk, Continue.
Otherwise, what's the point?

Things will stay the same or regress, unless you don't.

Communication is everything. That's how smart & conscious couples stay together.
Every relationship, actually. Whether its Bfs, family, jobs, friends, people you meet. Whoever.

Lots of people get into a relationship without knowing what it means to be in one.
Kicking back. Doing nothing but being angry. Blowing it off.

"I've told him to communicate with me"

Try another approach. A date to talk, seriously. Not flying off the handle.

He seems like the type of person who lets things build up before blowing up. If that's the way he operates, then you need to tell him that his way of approaching conflict can cause a lot of confusion.

Instead of simply telling him to confide in you anytime he's bothered by something, why don't you ask him what works best for him when dealing with problems.

First off he loves you and is afraid to lose you. Why? Because I've felt the same way about a women. We did have good communication but if I seen something that bothered me I would let it slide because I wanted to avoid confrontation. You should reassure him that it's OK to talk and tell you things so that it doesn't build up to the point of explosion. I know it sounds unmanly to need reassurence but the fact is that if he really likes you and doesn't want to lose you he's not going to be honest with you as the things are happening but will if you sit dowb and have that open communication. Hope this helps. I've been there...