A few variables here. A BT could weigh anywhere between 5 and 11 Kg, so 7 is either a smallish dog or a pup, I'm guessing here. Also, the amount of exercise he gets will determine how much he needs to eat. Age is a determining factor too. Having said all that, somewhere around 130 to 200 grams a day would be in the ballpark if he is an adult dog, a bit more if he's a pup. You should take him to your vet, ask him to weight him and give you an opinion of his condition, do it again after a couple of months and see how he's going. Basically keep an eye on his condition, you should be able to feel his ribs but not see them, he should have a noticeable waist. These are general comments, talk to your vet.