Well, many children's weight will dramatically slow down when they become more mobile; crawling, walking, etc. So hopefully you won't have to deal with this issue at all.
However, if you do, Cosco and Safety 1st car seats rear face to 35 lbs. I don't normally recommend them, but their Scenera
is a really good bare bones seat that will give you a couple extra lbs. The Safety 1st Uptown
is slightly more cushy than the Scenera, but still won't brake the bank, and also RF to 35 lbs.
If he goes over 35 lbs before his 1st birthday, you don't really have any good options. There are NO U.S. car seats that rear face over 35 lbs. You can try to import a seat from Sweden, where seats RF to 55lbs, but not only is would that be very expensive, but there is a ton of legal red tape to go through. Importing car seats from other countries is illegal unless you have a documented medical need, and while being over 35 lbs, and under 1 year IS a medical need, it's really hard to get it documented and get approval to import a seat.
You can turn him forward facing early, but that is EXTREMELY DANGEROUS because spinal bones aren't strong enough to protect the spinal cord in an accident at that age (nothing to do with weight or strength).
The only other option, also illegal, and potentially dangerous, would be to take your Britax marathon, have it installed by a tech, tether it rear facing, and brace it against the front seat, and keep using it after he's over the RF weight limit. Totally against manufacturer's directions, and not crash tested, and I would NEVER recommend this for a child who is under 35 lbs or over one year of age.
Maybe there are some options I haven't though of.... I'm not a CPSTech, just car seat obsessed. I'd highly
recommend asking at car-seat.com, or PMing one of the CPSTs that are on MDC.
*I only linked to "evil-mart" because they have the cheapest price on the Scenera, and are the only web site I know of that shows the Uptown.