My son was just like this - we have a 5 hour drive to and from my in-laws, and it was sheer torture. I remember one time I spent 45 minutes hanging over the back of my seat so I could wedge my head next to him and make "sh...sh...sh..." sounds in his ear.
Here's the few things that helped:
- Music. There are a ton of good kids song CDs out there, and even your own CDs might be appealing (email me for suggestions, if you like). The CD "No!" by They Might Be Giants was the one that worked for us, but we also made "mix" CDs. Try singing too - especially if Ava will join in!
- Toys - we brought a bag full of the small hand-holdable toys we had around the house. They usually only worked one time, and only for about 10 minutes at a time, but if you got them in his hands before he went too ballistic, it could stave off a full meltdown
- Breaks - keep up the breaks. Especially if the problem is comfort. Wrap the straps with some cushioning where they hit your son's neck
- Light - my son became completely unconsolable in his seat if it was dark outside. Watch to see if this might be part of the problem with your child too (my daughter ended up being very simliar). Try to schedule trips so that there's still daylight left (easier to do now than in the winter!)
- Bottle - this didn't work for my son, but my daughter likes having a bottle, so even though we're working on moving her to a sippy cup, we'd give her a bottle of water for long trips. We'd use a small-hole nipple so it takes a while.
And finally, don't worry - as other moms have said, this will pass. The hates-the-carseat phase was about 6 months long with each of my kids, and it will be over faster than it sounds.