I haven’t talked to my ex in 7 months but I am putting up my Christmas tree and I remember bringing ornaments over to his place last Christmas. Is it weird to text him and ask him if he’s seen any ornaments that I might have left?

No I think that’s more than fair. It is understandable you’d want those back.

I think it really depends on where you're at with the breakup and the implications of contacting him. Personally I wouldn't think it's worth it to re-open communication after that long just for ornaments because it would mess with my healing process. But if you feel like you've both moved on and it would be chill then go for it.