Decline in babies weight?

SL

So my little guy has gone from the 80th percentile in growth to the 40th percentile and has only gained 1 pound since his 4 month check up, he is now 6.5 months. Our pediatrician told us he is sleeping too much at night and missing at least 1 to 2 meals during that time. He sleeps from about 7:30/8PM to 6/7AM. She said we need to start waking him around midnight to feed him and will be surprised at what an extra feeding will do for him. Everyone tells me he looks so big for 6 months and he was spitting up a lot from around 3-6 months and of course he is way more mobile now. Just curious if any other mothers have gone through this and if I should truly be concerned.