I'm finding for cooking purposes, the Alpro creamy works well.
I also like it in coffee, but it's too much in tea - I've taken to having my tea black with honey instead.
I feel like if you're going to put oat milk in tea, you want one of the more watery own brand ones instead.
We're not completely off all dairy products, he's alright with hard cheeses as long as he doesn't overdo it, and yoghurt (so it's a lactose issue rather than lactase, or a milk allergy) and oat milk in general has been a bit of a game-changer in terms of him being able to enjoy a bowl of muesli or a white roux type sauce with something. That and soy Ice Cream
Now just have to suss out whether there are any tinned custards that are any good and don't break the bank, some of the chiller cabinet non-dairy options are mortal expensive compared to a regular tinned custard.