Almost 2 year old bp and only 345 grams?

Sounds like he was way underfed by the previous owner. If he has gained weight for you, he will probably continue too. Unfortunately the pet stores sell babies and tell the owners to give them a mouse every week or two. Well, some owners forget how many weeks have gone by, and when they think of it again it could have been 4 or 5 weeks.

Mice are not a good source of nutrition, they are not dense at all. Switch to the rats not soon, but now. Get that fecal done, just in case. Your boy will explode once you get him on the rats.

There is a chance he may reject rats. If this happens go get 6 mice, keep feeding them until he quits eating them. Then in about a week try a rat pup, he may switch then.

If you want him to gain more weight I'd suggest putting him on rats but from the looks of him he doesn't look unhealthy at all. You cant really compare him to others that you see because most of us (myself included) honestly overfeed our snakes in order to get them up to breeding size, so a comparison to a 1000 gram year old is no comparison at all. If he's eating well every week and you're having no other health problems otherwise then there's nothing to really worry about. If you want him to get bigger quicker then rats would be the way to go but don't worry about his size for his age. Some just grow faster than others too...I've had some that eat the same size meal every week and 1 would be twice the size of the other!