Mamas and Papas make a useful highchair insert that is not floppy like most but has wire inside to make it quite mouldable. Useful for not-very-upright babies (smallclanger could sit when he went into his highchair but he had a tendency to slouch anyway). I imagine that there are other manufacturers making a similar version but most I've seen are unwired AFAICT and just rely on the highchair frame to provide support.

With luck the growth from solids will be counteracted by the expended energy when trying to get mobile. :)