Why don't you let him decides, when he hits puberty? It's his body, and a 12 or 13-year-old boy is certainly old enough to be involved in healthcare decisions like that.
You can get mumps twice, no matter how many sides you have it on. Having it on one side provides just as much immunity as having it on both. Some people just lose their immunity over time. Actually, all people do, to varying degrees.
If you wanted to know if he was "REALLY immune", you would have his titres checked. However, even then, there are no guarantees. Someone with plenty of antibodies could still get mumps, under the right conditions. But sterility is extremely rare. It rarely (think NEVER) happens in both testicles, so he would still have one fully functioning testicle. Therefore, *he* would not be sterile, he would just have one sterile testicle. It's like how if you have one ovary removed, you're not sterile.