Hi Nikki and
to the forums!
I'm sorry to hear that Poppy is having trouble getting around! I agree that since it's just in his feet and an avian vet already said he is healthy, it could very well be a neurological issue causing the muscles in his feet not to work properly.
Julie has given great advice and I also think that along with a healthy diet and lots of TLC, making his cage more handicap friendly would be great. You could line the bottom of the cage with paper towels to cushion his landings, and put his perches close to the bottom of the cage so he can get back up if he needs to. Flat perches as well as some rope or wide wood perches may help his feet, too. Beware of the rope perches, though, if he's a big chewer you shouldn't leave them in his cage as chewed ingested cloth or rope can cause crop impaction.
Perhaps he suffered a stroke and has partially lost the use of his feet muscles, either way it sounds like you're prepared to give him the care he needs to live a full and happy life
Some members whose budgies have experienced similar symptoms after a tumour or stroke have reported success with knotgrass, a natural grass with healing and detoxifying properties, as well as giving them some chamomile tea (cooled to room temp, unsweetened) to soak their feet in, as chamomile is a relaxant and soothes injured or spasmodic muscle tissue as well as inflammation.
I hope this helps!
We'd love to meet your little boy when you get a chance!
Be sure to read through the stickies as well to ensure that you're up to date on the best budgie care practices. Feel free to ask any questions that arise afterwards!
Hope to see you around and please keep us posted on Poppy's condition. I hope his symptoms can be alleviated somewhat.