I have no advice, but had to tell you I hate the ice cream truck - they come when it's barely 60 degrees, and it's not just one time a day, it's multiple trucks and that annoying "hello, hello" with the music. OMG I hate them.
We taught our kids the economics of it - we showed them how much money it costs per ice cream bar at the store compared to the same things from the ice cream truck.
I think you just need to teach your kid to say he is not allowed to have it.
As far as the allowance thing - I don't give our kids actual cash but instead keep a dry erase board with their current money totals. When they want something we talk about it, and if it's something that is ok they can use their own money but I charge it and just subtract the amount. Since I have a 10 yr old they may sometimes go to CVS with some cash from that account, but again since I am the one doling it out I have control.
Guess I did have advice, lol.