he may not be a pure breed. If he doesn't look fat then i would just feed him as usual. Unless an animal is obviously obese or has a medical condition i hate to starve them as they have no other way of getting their food but through us. You could take him to another vet to get a second opinion stressing that you do not believe he is pure mini lop and therefore may be bigger causing the weight discrepancy . The weights are after all just guide lines and not chiseled in stone. I have never met anyone, man nor beast getting fat off of too many veggies. taking off 2kgs would be more than halving his weight which sounds ridiculous to me and harmful. Get a 2nd opinion if you are worried.
Mikala