Well, not counting before I knew better and reharnessed the boys...
Damian was (and is... he's 5'1" at 9.75yrs old... he'll be 10 in January) extremely off the charts tall... so he outgrew his REGENT at 6.5yrs old... when he moved to booster full time.
Jeffrey was bugging me about moving to a booster when Damian moved... and we made a deal... which I kept as he was indeed ready... he had to be 6yrs old, starting 1st grade (he was in Kindy and weighed 48lbs when we made this deal, btw), weigh 50lbs, be able to buckle himself (which he was already doing in his harnessed seats, so I didn't want him getting frustrated by not being able to buckle himself in a new seat), be mature enough to stay still the whole time, and prove he was going to be mature enough by not asking me again until he was 6.
He didn't bring it up again at all... and when he was 6, he was 50lbs (again, I listed that just cause I knew he would be... and it made it sound like he needed to be bigger than he was, not just older), doing all that other stuff, and I let him move to a booster full time.
Ruthie has been using a booster in her dad's car since she was 5yrs old... to booster train her... for the purpose of emergencies if she had to ride in some other car, for field trips and the bus (she takes the special needs bus, where they use seatbelts, but still, helping her learn to sit still with less restraints)... and because I knew she'd probably be outgrowing the Nautilus around age 6.
She's not got much room left and she asked when she could have a booster in Mommy's car like her brother... so I did similar to how I did with her brother... she's still 44-46lbs, depending on the day, so I didn't give her a weight limit... just told her she had to be 6yrs old, being a good girl in school (her birthday isn't as close to the new school year as his... he was July, so I can't add the 1st grade thing, even though I know she'll be going into it next year), be mature enough to sit still the whole time and sit properly even when sleeping, and buckle herself with help (the booster she's getting is the Recaro Vivo... and it's trickier than the old Parkway with no armrests I had for the boys, so she still needs a bit of help to thread it... she can buckle it, but then I or one of her brothers has to help her get the belt under the armrests and make sure it's snug.
I know she's ready and she'll do fine.
Now, for your son, I'd get a booster for grandma's car and start training... when he outgrows the Marathon, I'd get him a booster for that car... and after that, it's all about what you're comfortable with... but I like kids to be over 40lbs (unless extremely small) and at least 6yrs old... I bet your son could make it to age 7.