The Nautilus and Frontier have top slots approximately 1.5 inches higher than the Marathon; the Regent's top slots are 2.5-3 inches higher than the Marathon.
Where are your son's shoulders in relation to the top slots of his Marathon? My daughter was too tall for a Marathon (shoulders over top slots) at 46 inches tall, but your son may have a shorter torso and longer legs. This is very important to note, since a forward-facing harnessed seat is outgrown by height not so much at the written standing height limit, but when shoulders go over top slots or when tips of ears go over top of seat shell.
My daughter was too tall for a Marathon around 4 years, 9 months old, and she would have fit in a Frontier or Nautilus harness until about 6.5 years old. She'd fit in a Regent until about her 8th birthday, I'm guessing. So in general, I think the Frontier and Nautilus can last 1.5-2 years longer than the Marathon, and the Regent can last 3-4 years longer. There's a lot to factor in when deciding whether or not to continue harnessing or move to a booster, and a lot to factor in when deciding which option to go with. Hopefully knowing what higher weight harnessed options out there will work longer than your Marathon will help narrow down your decision. Post back with any other questions.
The vehicle (year, make and model) and current carseats/other children could influence what choice(s) might be better or worse, too.