Congrats & welcome to YT
Your little baby is a cutie! I agree that at 5 months, he may be teething. That can sometimes cause stinky mouth. You can ask your vet to examine his teeth. In the meantime, you can try Tropiclean Fresh Breath Water Additive. They have it on Amazon for around than $10. It's a liquid that you add to their drinking water to help reduce plaque & lessen the need for the toothbrush. I've used it before & I'm not positive if it helped with plaque build up. But it did help with mouth odor. Good luck!