Booster seat for child with special needs

My son is going to 5 in June. He has had one heck of a year with a tracheostomy and now leukemia. He also has Down Syndrome, but is functioning well despite all the medical crap. We have a Britax Boulevard in my main vehicle and a Marathon is my hubby's truck. We have to buy a booster for my mother in law's car. She is really funny about her leather seats.. (I have them in my car too with no problem) so the car seat has to come in and out of her car. She watches Nate 1-2 times a week and will need to pick him up from school and camp this summer with short rides from there to her house.

I am so torn on this because I know we will convert over to a booster eventually, but I want to still have the 5 point harness. He does have lower tone because of the Down Syndrome and does fall asleep in the car. I checked Baby Bargains and they are recommending the Graco Nautilus. I know the Frontier is coming out too. Should I buy the more expensive seat knowing that we will eventually use it in our car or will we get a lot of wear out of current two seats... I don't want to compromise safety for price... but we aren't rich....

His current weight is 46lbs and his height is 40.7 inches. His shoulder to hip is approx. 12 inches.

Thanks so much for your help

How does he fit in the Marathon? What slot is he on?

The nautilus is a great seat, that really should suit him if it doesn't install "too" upright in your mil's car. Some have issues with headslump in it, I have never had that problem though.

Now, if you really feel he will need long term harnessing, then maybe a nautilus for one car, and a new frontier 85 later on for your main car.

I was thinking Nautilus as well.

It has a fairly flat/smooth base so it shouldn't cause the usual 'carseat dent.'

It is an easy install in most cars.

The Nautilus will give you at least another 1 1/2- 2 inches of torso growth over an MA and makes a decent booster after that.