While I am not a tech (nor an expert), I think the first thing to consider is why he wants to unbuckle the seatbelt. From your post, it does not sound like physical support needs or sensory issues are the motivation for him to do so, but I would consider this possibility. For instance, if he is uncomfortable then he may be reacting to being so (please forgive me for my lack of knowledge regarding autism here!).
You may wish to investigate the following seats:
• Merritt Manufacturing:
The Churchill Booster (suitable until 175lbs)
• Columbia Medical:
2400 Spirit Car Seat (Only suitable until 130lbs)
• Columbia Medical:
2500 Therapedic Car Seat (Only suitable until 130lbs)
• Special Tomato:
Large MPS Car Seat (Suitable until 150lbs)
• E-Z-On Products:
86Y Harness (suitable until 168lbs) requires heavy duty tether anchor - see below)
As
Patriot201 has recommended, if a "seat proper" is not required or unfeasible, an E-Z-On harness or vest may be the solution. We use the 102 model vest (we have used the 103Z model previously) and contrary to the first impression of some people, after they have been used the first few times, they are no more complicated to use than a regular car seat. This will however require the installation of their
Heavy Duty Tether Anchor (Item A). This can be very easy or very difficult depending on the vehicle. There is a
pictorial self-installation guide here, if you are interested in the process.
If you are to use a vest, the most complex decision to be made is likely to be what mounting system is best to use. This depends on the vehicle, the possibilities regarding installing the heavy duty tether anchor, the chosen vest, the needs of the child, normal vehicle occupancy and the to need access the seats/trunk, etc. There are two main types of mount; the
Floor Mount which uses two anchor points and four straps and negates the use of the seatbelt entirely or the
Tether Mount which secures to the shoulder straps of the vest but requires a lap belt, or lap-shoulder belt in conjunction with a locking clip, to secure the hips of the wearer. The latter only requires one tether anchor; the former two.
Further to
Patriot201’s comments, if you could provide further details about the vehicle, it will help recommendations.
I hope this helps.