Well, it's important to keep them lean so that their joints can fully grow in. Extra weight can lead to injury when they are so young. It takes a long time to let them finish the bone and joint building they need to do to carry such a large dog. Really, for most Mastiff full growth is around two years...He"said going to be a Big Boy.
I admit, it's hard when they look and act like they are starving. Their eyes can melt your heart. But at 7 months, his weight seems about right. Look at his ribs...how many are showing. You should be able to see the outline of the last one or two...more, then he probably needs more food. The last two ribs outlined, then you are probably fine.
What does your vet say?