My almost 5 month LO likes to sit up straight with support. We have tried putting him in a high chair and he seems to be able to sit rather well in it. His neck is already stiff enough so his head doesn't wobble. In a stroller, he likes the back panel to be in the sitting up position but his body will slant to the side. Will it be bad for his back to be sitting up frequently and also in a stroller if he keep slanting to the side?