Hmm. It seems to me that a quad (4-wheeled) cart would be the way to go with a tripod, but maybe not right now. Can you take the front wheels off easily? (I'm not familiar with the HP quad carts.) If that works, I'd certainly keep the front wheels in case your baby needs them later, perhaps even trying them again after an adjustment period. Quads don't seem to be as "natural" as the typical 2-wheeled kind, but a quad could help take some of the load off of his remaining arm.
You may want to post on the neurological board as well, or you may want to reply to posts there to talk to other parents in a similar circumstance.