He told his mom that we are just friends even though she knows he is gay...

Honestly I would be a bit bothered by that too. If he's out to her, and you're out, why hide it from her?

 

Hey,

I will weigh in here. I think you are completely right to be upset. I would be upset also if this was happening with me. However, the problem has nothing to do with you and I suspect it is a little bit of discomfort and embarrassment on your boyfriends part. I don't think it is something that you should feel like he doesn't care about your relationship or is not proud of you.

Even though he is out to his mom, sometimes letting people know about a boyfriend makes the whole thing more real than saying you are gay and might be something he is struggling with internally, despite being out to his mom. I think you should support him and see what is really going on before starting to take it too personally and getting mad at him over this.

Furthermore, the fact that he doesn't want to be confronted with this suggests another level of discomfort in addressing the situation that probably means he isn't entirely comfortable with her knowing (again might have nothing to do with you, but might still be not 100% comfortable with being out). Everyone handles it differently.

 

Hi! I am, in fact, your boyfriend.
OK, not literally, of course! But... what you describe here is actually pretty much how I act around my mom.

She knows I'm gay. She accepts I'm gay. She can live with the occasional off-the-cuff remark about it. But we never ever discuss the exact details of it. Around each other, we mainly just pretend I'm totally asexual instead.

And yeah, that extends to my boyfriend. All of my friends know about us being together. Most of his friends know about us being together. No issues hanging out with everyone and everyone knowing.
But when it comes to my mom, he's just "my really good friend that I otherwise involve in all of my decisions and go on holidays with, and mention all the time".
Oh, I'm fairly sure she knows what's up. But she never brings it up, and neither do I, and it just became so easy never to move out of our respective comfort zones.

What helps here is that he's exactly the same when it comes to his parents. In fact, they don't even know I exist. Which annoys me, but I have no real excuse to be annoyed because I'm not any better. So, neither of us has the moral high ground.

Why don't I tell my mom?
It's definitely not embarrassment or uncertainty. My boyfriend is the awesomest guy I've ever known, and I have zero doubts I love him more than I've ever loved anyone.
A large part is just inertia. It works as is, and there is little to gain from upsetting the apple cart.
Also, there is the dreaded momquisition. If I ever tell her, she'll have all of these well-meaning questions. "How certain are you of this?" "Will you be moving in together soon?" "Since you aren't exactly living close together, will one of you have to switch jobs to make this happen?" "What about marriage or children?" etc. etc. etc.
It will be well-meaning, but it come all at once and with the speed of a machine gun. And with us right now taking the relationship as it goes along (for us both it's the first really serious relationship we've ever been in), it will feel like she thinks we have no clue what we're doing, or that we're being forced to make deep decisions, NOW!

So, in short: your boyfriend might be the same. He is probably not doing it because he doesn't consider this a serious thing. but it would be confrontational and emotionally draining, and feel like he's forced to make decisions right now, and the current situation might suck a bit, but it still works.

Now, I am planning to rectify this situation. And the best plan I know of is to just tell my mom together. I'll let my mom know my friend is in town and we' love hopping by for dinner. And somewhere during that, we'll go ahead and tell what's the deal. Spread the burden of telling her, and together, it's easier to deal with than alone.

So maybe that's a good start for you too. Maybe even just start of meeting his mom as "your son's good friend", for just a handshake when he swings by her place to get something. Then, work towards the two of you telling her what the deal is, and tackling this together. By the end, she'll know and you'll have even strengthened your bond through it!

 

I watched a situation very similar to this in Modern Family about an hour ago.
Season 1, Episode 13. Give it a watch.