Congrats on the potty training!
They say you can estimate adult weight by doubling the 12 week weight and adding or subtracting a pound. This is still an estimate, but it's fairly accurate. I would say that he'd be around 6.4 pounds.
I would buy the more expensive Petrodex toothpaste, it really does help reduce tartar and plaque and most dogs love the poultry flavor. You use so little, it last a long time! Right now, you just want him to get acquainted with the brushing action.