If his hypotonia is moderate-severe, he spends a lot of time riding in the car, or he sleeps a lot, I would probably buy another convertible, b/c it will give him more support due to being more reclined forward facing than the combo seats. He will enjoy car rides more and be less exhausted overall. If he's not having to ride a lot and it's fairly mild, the Frontier and the Prosport are more supportive and recline more than the Graco and Evenflo combos.
I'd suggest checking out the True Fit, Diono Radian, Safety First Complete Air, and even the Evenflo Momentum or Triumph 65. These all have a decent recline forward facing, a pretty nice shape and support to them for his size and hypotonia, and pretty good leg support (the more the legs dangle, the more pull on the trunk muscles). If he's really severe, then the MyRide is great, but to me, I don't like it forward facing otherwise. I think many 3 yr olds will feel *too* reclined in it, even one who needs the support might get sick of laying back so far.
We used a foot prop for my son when he turned forward facing for a long time. He has had mild hypotonia due to prematurity and autism...and definite preferences for certain seats!