My oldest son is now 8.5 and was in a Regent up until last year, his choice! He has mild aspergers and hated the feeling of the regular seatbelt, it wasn't constricting enough for him. When he decided at almost 8 he wanted a booster I bought a Sunshine Kids monterey and it was a great choice. Its headwings give him a sense of security and the seat is super comfortable and fits him great. He's almost 9yrs old, 4'3" tall and about 65lbs. Tall and skinny!
That being said, at 7yrs old a booster is appropriate for your son, but I get wanting to keeping him harnessed (my5.5yr old is in the Regent now and my 3.5yr old daughter can still ride rear facing!) the Frontier85 is a great seat, and you know your son best and how he will ride in a booster.
Another option is to keep him in the Regent and get your 4yr old a Nautilus. They are great seats too.