He has obviously found a single interest that really engages him… That is a very ‘seductive’ thing for autistic people.

One suggestion is that if he is dropping jokes that are not landing, ask him what he wants to achieve when he’s making these jokes… It might well be that he simply hasn’t grasped the fact that not everyone speaks the same language that he has discovered.

The world is such a puzzling place to autistic people at the best of times, and it really is possible that he just assumes that everybody else understands these memes in the same way that everybody else seems to understand so many things that are a mystery to him.

Maybe he’s found that memes give him a voice he’s never felt he had, a language he’s fluent in? Try communicating with him in memes and see what happens.

Don't try to connect from where you are. Reach him where he is. Learn his language, and learn to think like him, and don't force it. Remember that it is as hard for him to connect with others as it is for you to connect with him, that doesn't mean he doesn't want to or care.

“maybe find a better time to say the jokes so that they land better” and “please try not thinking about memes all the time”

This is not advice, this is telling him off and he will simply discard it as it is pointless as telling someone with asthma to "just breathe better".

Verbal communication is simply annoying. There's no time to reflect, you have to get the words out of your mouth, your brain will be ahead of what you're saying. Reading and writing generally have less ambiguity and are preferable.

How old is your brother? I've been hella awkward until about my mid 20s (there were complicating factors delaying my social development) but I'd like to think my social gaffes are a lot less intense now. AFAIK many autistic people adapt a little more with age.

I like what others said about learning how to communicate in memes to connect with your brother. Memes might be his native spoken language. It can help him a lot if he isn't the only one speaking that language.

The tips you gave him probably don't make sense to him. Trying to find a better time when he probably doesn't understand social context well might have gotten translated to "wrong time of the day, try this one after 1pm". If he is interested you could explain social context to him on a case by case basis an explain why sometimes things are funny and sometimes they aren't. But it's exhausting work.

If memes really are his native spoken language you asking him not to think in memes might have sounded to him like "don't think in words" (because he doesn't know another way to think) which is a confusing request and he probably only agreed to make the confusion go away.

If he is like me (e.g. talking about only one topic like tech, gaming, flavor of the day) i let him be. Caring is good. At one point he will either out grow the topic and it will be a side interest or outright be done with it.

This is assuming your brother being young. If he is older, try to steer him to other topic or let the guardian do it.
Maybe he needs to see a topic that peaks his interest.

This might not be great advice, but you could incorrectly use memes all the time in front of him to the point that he cringes into not using them.

Get him into anime

I think his preferred method of communication is through memes. He might find it validating. Perhaps, you can try talking to him via memes too.

I know I love memes, and I send them to my friends even if they don't send me any. What I'm saying when I send one is (1) this is something that I relate to, (2) this made me think of you and that's good because it means you're important to me (penguin pebble), (3) I like sharing things that make me happy with you because I want you to be happy, and (4) I feel safe being open about this with you. If he's anything like me, he has a secret stash of memes he's holding onto until he feels safe and connected enough to someone to share them.

Limit screen time

This is just discord addiction.