Yeah my little boy started in his jumperoo at 4 months, he couldn't reach the floor but I just had to pad up his feet so he wasn't dangling with all his weight on his hips. 1 month on and he can reach the floor and he's figured out how to bounce, we'll be moving the height up to level 2 soon.
We started tummy time as early as possible too, even if it was just on my chest to start.
They recommend building it up slowly and doing 20-30 mins a day by the time they are 3 months old. No wonder he has trouble turning his head if he's not getting tummy time.
I think they've been soo preoccupied with the feeding issues they've forgot about anything else.
i just write my experience in hope they read here. But then they should get advice from parents and sister if no one else