My sheltie dosn't like walking after its been raining, he gets neverous of the wet tires on the road. I find the best thing to do is to walk normally and just use a firm "heal" when required and lots of praise when hes walking nicely.

Yours is still young though and I think with heal trainning and practice he should grow out of it.

You can also try turning the other way to desctract the dog, or making it sit

Good Luck!