Hi. Depending on your sons weight, his car seat may not be all that much bigger than a standard seat. This is because the adapted seats are based on the same designs as an “off the shelf” seat with adaptations added. JCM produce very good seats of you need it to swivel although they are v expensive. Have a look at this site for potential help funding your sons’ seat: :https://www.wellchild.org.uk/2018/08/10/extra-support-grants-families/
Hope this helps, good luck with ur car search.