Definately normal
No worries lol
If you bath him, it will help loosen all of the undercoat for it to come out
He isn't fully "blowing" his coat though, its more then likely just the adult fur pushing its way through.
Chime at 4 months is starting to grow her adult coat in, she has a "skunk stripe" down her back with new fur lol
Crosbie and Jada went through the same thing
No need to take him to a groomers for it, unless its out of control lol
Just give him a nice bath, dry hiim and then groom him out
It will grow back! lol Thicker and Fuller!!