My 3mo baby's weight has remained stagnant for 2 weeks. Should I be concerned??

My baby has been on the smaller side since birth, btw 10th - 25th percentile, generally has been gaining weight each wk prior to these 2 weeks. He's fully on BM, was latching him on demand previously but recently started to feed him via bottle during the day to prep him for infant care. He takes the bottle well, doesn't reject (~135ml), feeds 6-8x, approx 6 wet & 4 dirty diapers a day. Poop colour is normal, mustard yellow. He seems pretty fine overall, able to smile and laugh. Only thing is he doesn't nap well in the day because he wakes up rather quickly after being put down. And we are trying not to carry him to sleep. He gets around 14 - 15 hours of sleep a day in total. As a FTM, I am not sure if this lack of weight gain should be a concern?? :(( any advice??