I would keep him in the tank! As the others say, Syrians need minimum continuous floorspace of at least 32" by 20". How big is the tank? If you really want a cage rather than a tank there are a few options that are a suitable size in the US (just a handful!) I also think it's not a good idea to downsize, unless he is getting old and you think he needs a smaller environment. If you move him to a cage smaller than his current tank he might start bar biting as it could feel too small.
There are a couple of European cages available on Amazon.com - but they're not cheap. One is the Savic Mickey 2XL - About 32" x 20" - I had that cage for our Syrian and personally feel it's not quite big enough once you get all their stuff inside. I just upgraded ours to a 100cm cage (about 39.5") which is a good size for a Syrian I reckon.
Of the US cages available, there is the Marchioro Goran, which is a rat cage but has 1cm bar spacing so fine for a Syrian - it just meets the minimum recommended Uk size. And there is a 100cm cage available on Amazon called the Rat and Ferret Kit Cage which is about the only big hamster cage available in the US I think. It's the same as the Mamble cage in the Uk. Bar spacing is fine on that too, but it is very tall and could really do with a full level putting in it, or quite a few overlapping shelves, to prevent injuries from falls from a height. If you are good at diy you could make an extra level for it - otherwise it means buying extra shelves. Currently it's at an extortionate price!
Ferret Kit cage 39"
Amazon.com : A and E Cage Co. Ferret Kit with Tubes : Toys And Games : Pet Supplies
There's a smaller 31" version but you'd be better off getting the Goran/Kevin than that, it's a better cage.
The Goran doesn't seem to be sold by Dr Foster and Smith at the moment and is the same price as the Ferret cage on Amazon. So both very expensive at the moment!
Amazon.com : Marchioro Goran 82 Cage for Small Animals, 32.25 inches, Beige/Green : Pet Cages : Pet Supplies
I think most US hamster owners reckon about a 40 gallon long tank is the minimum size for a hamster really.