Maybe use a clicker before you toss it?
Pretty much this. Alternatively, instead of a clicker you could substitute a marker word such as "good!" or "yes!".
'Where' the dog is focused at the time you use that word or clicker, is the important thing. Well, it's usually at least one of the criteria. Other criteria may be the dog's position (ie - heel / in line with your ankle or a particular part of a wheelchair) or the actual behaviour itself (ie - leave it / the exact moment of relinquishment).
Also, I would try using very thinly sliced chicken wieners as the reward. Freezing them before slicing will make them much easier to slice, at approximately the thickness of a dime or slightly less. Use of wieners (now thawed) will reduce the tendency of the treat to 'bounce' when it hits the ground, which will reduce the likelihood of the dog needing to sniff and search for it and potentially lose his focus on you. The treats will basically sort of 'stick' wherever you toss or drop them, which should aid in a faster focus recovery time, and aid in establishing precise positioning as well.