Baby Constipated from Supplementing

My son is almost 3 months old and I exclusively pump for him, I have started to supplement with some formula (with the doctors okay) and he pooped last on Monday and he hasn’t pooped since, he keeps passing gas and doesn’t seem to be upset about being constipated or fussy at all so I don’t think it is bothering him but I’m wondering if any of you have any tips or tricks for getting him to poop. I know it is normal for babies to not poop once starting formula but it still worries me.