Disappointed 4 month check up

My son isn't gaining enough weight, he went from 90 % to 50 and now 20... doctor wants to add two bottles after breastfeeding to see how much he'll take.

Also, he uses one arm more then the other to grab toys. He's not interested in rolling (18 weeks today) he loves grabbing his toes and turning to his side though..

And doctor is concerned because when he's on his stomach he doesn't stick his arms in front of him. He keeps them on his side 😞

Other then that, he's a super happy wiggly baby!

I feel like I'm failing as a mom 😭

Is he just developing on the slow side?

Doctor wants him to see a neurologist and go to therapy.