When DS1 was an infant, I had a friend with a little girl born the same month. I believe her little Gracie weighed about 10 lbs at 6 mos, if I remember correctly. She was ebfing her, and her ped was totally freaked out and pressuring her to supplement with formula, yada, yada. Although she had very little support, my brave mama friend persevered, trusting that her babe was okay, just genetically programmed to be small. She paid careful attention to her DDs milestones and dirty diapers, but not to the scale.
We moved when the kids were 3 yrs old, and I haven't seen her since then, (it's been a coupla years), but at that time, Grace wasn't so little anymore. She looked exactly the same as all her little buddies that were born that fall of 2000.
If your mama gut tells you something is wrong, then pursue it. But if you and G were on an island with no other babies to compare to, and no scale to weigh him, would you think anything was wrong?
Oh, and to answer your original question, Will is 15lbs and change, probably the biggest of my 3 at the 6 mos mile marker. He is not yet doing those gross motor activities you mentioned in your OP, either.
Hang in there, mama. We love you, too!