Were you able to get the information? A lot of people don't realize they have latch connectors, but they're in every (I think) car from 2001 or later and many before that. We had a 1997 Ford Escort that was very easy to retrofit.
If you determine you don't need the Britax for adequate growth space, you could aslo consider the Harmony Defender. It is similar to the Graco (similar stats and has the three in one: harness to high back to backless booster conversion) but runs around $100.
You'll also want to consider how long you want to keep him harnessed, If only for another year, you don't need as large a seat as if you think he will need harnessed for several more years.
There is no evidence that a 5-point harness seat is safer than a properly fitting booster for a child of that age. There are arguments both ways for safety (some think a booster is safer because it gives more and so has less chances of head excursion; others the harness because it provides more security). Since you can't rear-face, your son is already perfectly safe.
It won't "become" fused, it already is fused. The foreskin is fused to the head (at birth) much like your fingernails are fused to the nail beds. With time (and hormonal changes, eventually) that fusing will release over a period of years (3-18 years of age) and no longer be attached in the same way. If artificially detached (like during circumcision or forced retraction) the foreskin will attempt to self-heal and refuse--- parents are advised to prevent that. ...
Apparently this is what Trader Joe's and some other companies that have traditionally offered insurance benefits to part time workers are doing--- cancelling the insurance of part time workers but, in the Trader Joe's example, the average employee will save over $1K yearly on the "same" insurance PLUS Trader Joe's is giving them $500/year to buy the insurance so they're coming out way ahead.