
1. I have to ask why you have a 6 week puppy- it is illegal in many countries now to sell puppies of under 8 weeks, unless your family bred him?
2. Why is he running a fever - so needing antibiotics? Is there something else going on with him - eg Parvo? because if you did buy him at such a young age, this smacks of BYBing so it's quite possible he came to you with this often fatal disease
3. Reading your further comments, so your family DID breed him - feeding from mum..... which begs the further question...
4. Why? - at 6 weeks he should be pretty much weaned onto solids, so there's no need to be involving poor mum (sharp teeth)
To be honest, I'm not sure I believe any of what you are saying. Went under the baby gate and fell down the stairs? What about the others in the litter .... or was he a singleton puppy? Our 6 week puppies were still in a big whelping box, or out in the garden in an ex-pen. Not able to get anywhere near stairs for sure.
I guess, if you can't have an MRI done, you'll just have to give it time and hope - high temperature apart, puppies do tend to 'bounce'. Personally I'd be more concerned about why he has a fever.