Sorry to go on about ABA, but this is one area where the ABA tutors saved my sanity by getting my boy to walk for me (we had exactly the same problem as you, he simply refused and wanted to be chauffered even 100 yds down the road to school).
I think you don't even need the ABA tutors themselves, but could try the techniques, as it worked for me too.
a) Insist he walks somewhere when you know you have a bit of time (eg don't necessarily start this on school run)
b) tell him you are going to walk for sweets, or ice cream, or whatever he really finds motivating. Make it a big , exciting thing
c) Once he drops to the floor tantrumming, hold his hand firmly (so he can't hurt self or get too near pavement with head etc) and look away, no eye contact, no words , nothing
d) wait out the tantrum (this is hard, and you may get looks and "tutters" but it's important you show him that he will get absolutely zilch reactions from you until tantrum is over. Think, as you go through this pain, that you don't want him in a pushchair when he's 13.
e) Once the tantrum starts to abate, but only when he has become much, much quieter for a minute or more, say "let's go for swees/ice cream then" and march off happily down the road, holding his hand and giving him eye contact again
f) be prepared to go through the step d) again and again, and keep repeating "let's go and get sweets" when it's over.
g) make sure he gets the the sweets by end of exercise. You HAVE to wait him out, you have to teach him that your will and patience levels will always trump his tantrums. And he has to learn that the only way to get the reward (make sure it's a very motivating one, ice cream always worked for us) is to walk nicely with mum to the shops.
It was slightly helllish to go through at the time (he was just under 4, same age as your boy), but I swear it took only a week and he now walks everywhere quite happily, around town , on the beach , to school etc.
Actually, they call it ABA but it's not that different from something I saw on Supernanny.