How old is your son?
When you say a dip in weight gains, you mean he's still gaining weight but not as much? It is normal for their weight gains to slow considerably as they get older.

That's quite a lot of milk - it's possible that he's filling up on it and just doesn't have room for solids. Even when babies breastfeed very frequently, they naturally take less and less milk overall as they eat more, it's quite a neat system. But when we replace feeds with big bottles we can inadvertently add more milk into their diet.