That sounds perfectly normal to me.
I don't start solids until they are around a year old, and mine are exclusively breastfed. They still grow well and are not skinny or anything like that. They are healthy little kids.
Breastmilk is so easy to digest that they will nurse more often than a formula fed baby will take a bottle. Being easy to digest is good - and healthier. He will manage to get more nutrients out of the breastmilk.
I also worry about babies who are sleeping through the night. It's not natural or normal. Babies need calories during the night, and they should not sleep heavy. They should sleep very light, as their REM cycle is much different than ours. This is because of their rapidly growing and developing brain. It's a necessary part of brain development for them to wake during the night, and for them to need calories during the night.
I wouldn't worry too much about whether he is getting enough, as long as he is growing normally. It sounds like he is getting plenty, IMO. He'll taper off as he gets older. I nursed my daughter more than a year, and at a year old, they do taper off and start sleeping longer stretches during the night. By 17 months old, she was sleeping about 6 hours straight. By two, she started sleeping 8 hours straight, and sometimes longer. By 3, she started sleeping 10 straight hours. I think this falls in to the normal range for development.
Take care!
Teri