Ds1 gets bad motion sickness in some cars, but not in others. We've found that getting him to suck a mint usually works with him for short journeys, but longer ones still require regular stops and 'fresh air' breaks.
On buses, he's actually better in the middle than the back or front, I assume its because its more stable and 'swings' less, but I know as Al1son said, most people are better off right at the front where they can see out. Ds doesn't get sick at all in our VW camper because he's high up and can see out really well, but is at his worst in low slung, sporty type cars with restricted rear vision.
Stugeron didn't work for either of my dses on the ferry (they both get seasick and we travel by ferry every summer) but the old fashioned Kwells worked like a charm for them both last year.
Boots also sell the Nelson's Travella homeopathic ones. They contain the homeopathic remedy cocculus, which is what cured our extremely travel sick dog years ago. I was sceptical, but it certainly did the trick for her. (We gave her straight cocculus, not the Travella ones).
The only other thing I can think of his aromatherapy. I know some oils are supposed to help to reduce nausea - I've only had a quick google, but came up with this.
Oh - just had a thought. Have you had his sinuses and ears checked at all. I still get motion sickness now if I have anything going on in the ears/sinus department. The GP may possibly be able to offer some better medication as well.
Poor boy, there's nothing worse than motion sickness, I suffered myself as a child and again through each of my three pregnancies - not a good way for him to have to start the school day every day. Hope you manage to find a solution for him before August.