Ex bf and child birthday parties

So my ex and I share custody of my son

Lately he has been getting invited to so many birthday parties and they always fall on my weekend with my son which is fine but it’s starting to get spendy. I haven’t asked my ex to help buy some of the birthday presents for the kids cause it’s always been my weekend. Well today I got 3 more birthday invites and once again they’re on my weekend. Is it wrong to ask my ex to buy one of the kids presents? I know I only need to spend a little but honestly, it’s adding up when you live pay check to pay check. I could just say we’re not going but my son needs to get out and socialize more so this is a good thing for him.