I have a magna and found that I could not have the capsule in the middle next to a maxi rider, or the capsule on the side and maxi rider in the middle. They pushed them sideways. Not sure for your one though, and the capsule I have is the unity safe n sound. You might have to just see if they can and if not get them to redo your sons seat at the same time.

I have mine with the maxi rider behind passanger, and capsule behind driver. Dh can just get seat back far enough to drive. I did this as the maxirider has my 6 year old in it and my 9 yr old sits in the middle (to me she is more protected this way) they get in and out on their own and I get my 15 month old out hence why on drivers side, she is stil in the unity capsule.