seat for bicycle

Hi, Just wondering if anyone can help me. I'd love to take my 20 month old son out for spin on a bicycle. He has spastic diplegia and can't quite sit up unsupported yet, but he's getting closer. I've asked the physiotherapist and she says there's no reason why I can't take him out. Does anyone have experience of this, and could you recommend a certain brand/type of seat for him? He has a twin brother, and my husband and I would love to take them out! Any advice greatly appreciated.