How sure are you he's purebred?
Also, if he was underweight his entire life then it takes a while to catch up. You have only had him for 2 weeks so have plenty of time to calmly begin feeding a good balanced and healthy diet so he can gradually catch up.
Growing TOO fast (adding fat not muscle) causes HUGE issues and can actually promote hip dysplasia and pano which you do NOT want.
So what are you feeding him? The amount on the bag is always a good place to start. He eating well? Happy? Full of energy?
We all freak out and worry worry worry about our pups growth the first year. But the best thing is to pay attention and be aware, but also look at how the puppy is behaving. If they are growing, are behaving normally with play and bright eyes, then we need to just keep up the good work and let nature take it's course.
MACH3 Bretta Lee Wildhaus MXG MJG MXF MFB TQX HIT CGC TC
Glory B Wildhaus AX, AXJ, XF
plus Miss Osin Blue Wildhaus
"Nothing new can come into your life unless you are grateful for what you already have. ~ "--- Michael Bernhard, gratitude