M.P.
You are right, it is MUCH safer to keep him in a 5 point harness until he is at least 40 pounds or more - depending if your car seat has a harness weighted to 40 or above pounds. As a Child Passenger Safety Technician and Instructor, we see kids put in booster seats too soon. Offering the 5 poings of restraint with the internal harness of the car seat is what is suggested. After that, as long as you follow the instructions of the integrated seat in your Volvo, he is as safe in that as a typical booster seat. I am guessing by what you wrote that the integrated seat can also be used as a booster...Safety is always worth the trouble and the tantrums that sometimes come with it :)