Can’t you give them the box to share? It’s an unspoken rule in our house that most stuff that comes in will end up being communal. So a colouring book might be your own, but the pens are to share. Yo can build your Lego kit first, but then it goes into the box so everyone can use it. And what goes around comes around, it means that each sibling gets to use more stuff, have a wider variety of things to use/do and they both see the benefit in that. They havent ever questioned it to be honest.
I think older siblings can have a rough deal. I’m a younger one myself so it’s not bias. I can see how from a very young age, they go from being the baby to having to do stuff for themselves, precisely because of the siblings arrival. I remember my daughter at 18 months, with few words, gently holding my hand and leading me back to cot and pointing to it, basically saying ‘mummy for gods sake put that thing down and give me a cuddle’. I have always remembered it, it was such a gentle but emotional plea.
The older one gets ‘you have to be grown up’ drip fed from very young, often is the less cute simply because they are older, gets the rules tested out on them first, is th one we make our parenting mistakes on. I completely see how they might be jealous, resentful, a bit fucked off with the world unless handled delicately. I don’t always get it right here but I try to keep an eye, and when I see she is getting fucked off, love bomb a bit, explain I know being the older one is tough, and most importantly, keep an eye out for the younger one deliberately pulling her strings or trying to get her in trouble. I did it, because sometimes it’s all your have in your armoury as a younger (slower, in every way) one, but it’s next right or fair on the eldest.
In your case, bearing in mind what you’ve said, I would either get them to share it or I would get him a little something to say ‘you’re important too’. If he is struggling, this is not the time for a bloody life lesson t life is often unfair.