Just because you don't understand something does not mean that it holds no value
Anyone who says "All gaming is childish and has no benefits" is guilty of exactly the same thought process that leads people to say things like:
"All Muslims are terrorists"
"All Immigrants are benefits scroungers and stealing our jobs. At the same time."
"All Women hate football and all Men love it"
You're just wrong. Flat out wrong.
You're ignorant, and you don't care to educate yourselves, and you'd rather dismiss something as worthless than try to understand how it adds something to a person's life.
Gaming is not for children. There are games FOR children, and there are also games rated 18+ which, last time I checked, made someone an adult.
People play games for many reasons. For the satisfaction of winning. For a feeling of accomplishment they don't get in their day to day lives. To be able to see themselves making definite, measurable, quantifiable progress towards an achievement. To make friends and meet people. To talk to other people who have the same interests as they do, since their DP and colleagues at work don't give a shit. To enjoy a story where they play the main character, and choose how the story ends themselves, rather than being a passive audience like watching TV or reading a book. To enjoy the challenge and difficulty. To get the adrenaline glands functioning after a day of mindless grunt work.
Those are just off the top of my head in 30 seconds. There are more reasons to game above and beyond those I listed.
So if you think gaming is a waste of time, I suggest you shove it, and go back to staring at the TV to catch up on Bake Off or Corrie or whatever other programs you like.
That being said:
Anything that disconnects you from your family on a daily basis is bad.
Time spent together as a couple without your son being present is important. It's healthy and beneficial to be "Tom and Jane" rather than "Mum and Dad" every once in a while.
Gaming is not the problem. Your partner disconnecting from you is the problem - whatever the vehicle is that allows him to do that. He needs to understand that relationships that are ignored are relationships that break down. You have to maintain it, just like you maintain a car or anything else. You need to make it clear to him that this behaviour is potentially relationship-ending. If not today, then maybe tomorrow, or next week, or next year.... but it is going to end the relationship at some point. Even if it doesn't, it's making you unhappy every single day. If he gives a shit about you and your feelings, he won't want that.
If he's an adult, then he'll be open to a compromise. I game when my partner is not around, or when she's busy doing something else. The moment she's around and available I ask her if she'd like to do something together, or if she wants to do something on her own, while I play on the laptop. If she wants to do something, the laptop shuts down and I join her. If not, we do our own things happily. He needs to have some kind of system where he is able to devote at least some amount of time and attention to you, and you to him in return.
So he needs to understand that. On the other hand, it seems like you have nothing to do apart from watch TV, which may not be the best thing for you. Of course you're tired from looking after your son, and just want to flake out, but is there nothing you can do? Knitting? Do an evening class once a week? Art of some kind? Writing? Having your own hobby will make it a lot less lonely when he's busy doing his. It also might help him feel less like you're picking on him just because you're bored and have nothing to do. From his perspective, he's doing something he enjoys, and if you don't have anything for yourself to do, that's your problem, not his.
I hope you manage to find a system that works.