I think the more people use the higher-limit harnesses and explain to their kids why, the less of an issue it will be. Teach your son to let any comments his friends might make roll off his back. I currently have a seat that is rated til 65lbs FF and DD will be in it til it either expires or she outgrows it. She will not be the only one in her peer group, as many of our friends have similar seats. It didn't used to be popular to keep kids in the back, either, but our kids adjusted and they knew that we simply would not allow small children in the front and risk injury from the airbags. Once it became the house rule, the kids and their friends honestly accepted it.
So...that said Babies R Us often has a trade in event. You can also look online for deals. One of our seats was bought at the end of the year but the two seats are the same model and will expire in a similar timeframe. You are unlikely to find a seat in the $50 range, but you might want to consider the TruFit, which friends have their nearly-5 yr old in: http://www.diapers.com/p/the-first-years-true-fit-convert.... It goes to 65 lbs.
If color is unimportant, you can often find a deal.