I got a new guinea pig two weeks ago and when I got him one of his tooth was about a third the size of the other. We found out later he has a lack of VitC which could be why his tooth broke.
But anyhow, Guinea pigs teeth grow constantly as you know. So yes the tooth will grow back in time. It took my guys tooth just one week to grow back fully (it could even be before this, but after a week i checked him again and it was fine)
But I did do infomation and it can take up to three weeks. But the thing is yes it will grow back.
The problem is the other teeth may need to be trimmed at a later stage as there is nothing wearing them down, as the missing tooth should be doing this. This is no big deal and easily sorted. Just if you notice the others being long then go to your vet.
You won't have to handfeed more than likely. As long as he is eatting fine. If hes having trouble with pellets you can crush them though it is better to have them munching on the hard food to help wear down the bottom tooth.
Also with the other tooth being loose aswell you may need to get the vet to check out the guinea pig, it could have a gum problem, or even needs more VitC. Do the gums seem red or imflammed?
Oh and if hes not eating all of his veggies during this time you may need to supplement the vitC.