Is my baby a slow weight gainer??

My LO just turned 4mo and only gained 3.5lb. I'm mix feeding and 3weeks since I started formula. I'd give him 2-3oz twice in the morning and afternoon. He'll take breastmilk in between but unlike before he doesn't nurse longer. I think he's too lazy to suck that's why I don't give him bottle that much, I'm scared he'd not want to take from the boobie. Please help me😭 should I increase formula? Since I started formula my milk ducts started to clogged since its not emptied efficiently but if I wait for him to be so hungry he'll not have the patience to wait for let down.