Harnessed booster for 7y/o w/ SN?

Hi all,
Newbie here. About to have my second child and have been enjoying using the forums to research which infant car seat to buy, will likely be the Chaperone, Be-safe or maybe the KeyFit.

I'm trying to find an affordable option for a pair of booster seats for my 7 y/o DS, 41lbs, 44" tall. (We have Frontier 85 and are fairly pleased with it - I lusted over RadianRXT but just haven't been able to justify buying a new seat when the one I have is perfectly fine. ) DS has a chromosomal disorder that has caused neurological impairments, cognitive deficits and some mild physical disabilities. Specifically, high functioning autism, hypotonia, nervous system problems, epilepsy and sensory issues make him a terrible candidate for belt- positioned booster seats. We've had to use a seat belt based boosters two or three times but he slumped, slouched and fidgeted the whole time. I think he is a couple of years away from being able to safely ride in a booster using a seat belt.

I will likely need a c-section so when the baby comes, my mom and MIL will be coming to town to help out. They both will need a seat to carry DS to his therapies and to/from school. I'd really like to stay under $150 per seat and one that he can use till at least 57" that we have on the Britax.

Any suggestions?

Thanks!!
Kirsten

What is he torso height? You say he needs a booster, but also say a booster won't work, so I am assuming you are looking for a Combination seat. Depending on torso height, I would look at the Graco Nautilus and Evenflo Securekid.

What is his torso height?

Could you just put the Frontier in your MILs car?

Sorry if I was unclear. I want a combination booster that can offers extended harnessing, like Frontier, which I believe is 57" in height. He is not capable of safely riding in a belt positioned booster.
I don't know his torso height but will measure it tomorrow.

When he has visited them (like over a weekend) DH or I have moved the Frontier to their cars so it is possible but it just seems like a hassle to move the seat 2 or more times daily. Plus I don't think I will be able to move it for a while after the birth and i just don't trust their abilities to install the seat on their own. It is everyone's preference for each of them to have their own seat - I'm just hoping to find slightly more affordable version of a Frontier or Radian.

Just so you're aware, the overall height guideline doesn't mean much compared to seated torso height. He may have more or less time than that based on when his shoulders go over the top slots-that's when it will be outgrown.

If you sit him cross-legged against a wall, mark the top of his shoulders, then measure to the ground that gives a good torso height estimate. Or, if you tell us what harness setting he's using and how much room he has before his shoulders reach the slots at that setting, we can make a reasonable guess.