You could get a leg-supportive backless booster (Evenflo Amp is a fave for big kids) and a Cardiff Headrest for the long trip, maybe?
http://www.amazon.com/Cardiff-CBL100...ywords=cardiff (I found mine on Zulily much cheaper...).
Is he 60lbs yet? The Incognito is really good for three across (Must be out of stock, that happens from time to time, never pay more than $19.95 for that one, I even got all three of mine open box buys for about $15...)
http://www.amazon.com/dp/B00FLXBHIU/...ords=incognito
A really nice big super cushy seat is the Diono Monterey, which also becomes a great backless, or the Recaro Performance, which does not.
Anyway, yeah, a backless is fine for an almost 8 yo.