I'm not sure who they'd get the information from. There's no "central database" for you to sign a medical release form to let them get your information, that I know of. The only possible way I can think of is if they can contact his previous insurance companies to get a medical history, and I'm not sure that'd be likely since that could be anybody.
However, if he did lie, and they ever found out, he could lose his job, lose all insurance benefits and possibly even have to pay back any insurance claims that were paid during the time he was insured (pre-existing condition issues, maybe even legally a fraud case). And god forbid he get hurt on the job due to this condition; he would have absolutely no way to claim workers compensation, nor would insurance pay for anything.
If the condition would affect his ability to do the job, then he shouldn't be taking the job. If the condition wouldn't affect it, then it shouldn't matter, and it's illegal for them to not give him the job due to the condition.
And again if the condition ever becomes an issue, the insurance company will not pay any claims for it as a pre-existing condition, and they will be able to find out if he was ever treated for it before (it might even be obvious if it's something that doesn't just appear suddenly). If they take awhile to figure out that it was pre-existing and do pay claims for a period, he would have to pay back any benefits that were paid out. The only way that pre-existing conditions can be covered usually is if he was continuously insured by his old insurance company up to the beginning of coverage by the new one, but then he'd have to have admitted to the problem in the first place.