Not jumping on the bed??!!

OK...this is probably a crazy question/concern, but why is it my 6 month old 56 lb puppy NOT jump up onto the bed. It's not that he doesn't want to come on it, He uses his paw to let me know he wants to be assisted to get up onto it, he can jump off of it, and when he is truly motivated (food), he WILL jump up onto it. I feel like he SHOULD be able to jump up to it with no problem. If I try to encourage him to get up on his own, he will just bark or continue to paw me. He is pretty tall...he can rest both front legs up on the bed when he is on his back legs)

I ask becasue I worry....are his hips or rear legs weak? He is a rescue, pup born from a pregnant puppy mill golden, so I have no family history.