Again, thanks for the reply. What you've suggested is basically what I'm doing. He doesn't play fetch; the only toy he shows any interest in is a food ball in which I feed him his kibble. But, I've been trying to use treats in the ways you suggested. I'll throw one 10 feet or so in front of us, and keep doing that down the block. Or, I'll have a few in my hand hanging right in front of his face, and if he is walking in the heal position I'll let him grab one every 20 seconds or so.
Mainly what I'm puzzled about is that this is a new behaviour and I don't know what caused it; he was the perfect on-leash walker up until recently. I could bring him anywhere; leave him tied outside a shop, or even bring him in if they allowed dogs inside. Do you think it's more likely that he has adopted this new behaviour as a result of feeling "off" from his infection, or that he's lived with me long enough to know that I'm not going to hurt him if he makes me mad (as others have done in the past, no doubt) and so he's trying to push his boundaries with me? It seems to me that I might want to approach the situation differently, depending on what it was that triggered it?
Also, sometimes if I walk really quickly and pull him along with me, eventually he'll give in and not resist. (He wears a harness, so I'm not hurting his neck.) However, I'm reluctant to do this, unless I'm trying to get home and have run out of treats, because I'm not sure if it is counter-productive or not?