Residency determination for J-1 visa holders is a bit different from other visa holders.  Could you clarify what you mean by "meets the residency requirement for tax purposes"?

If your client no longer qualifies as an exempt individual and meets the substantial presence test, a 1040 should then need to be filed.

In the event your client no longer qualifies as an exempt individual and is a resident alien, he would generally be subject to FICA since the exception under §3121(b)(19) would not be applicable.  But if the wages received are from on-campus employment at the university in which the individual is enrolled and regularly attending classes, the exception under §3121(b)(10) and §31.3121(b)(10)-2 would still apply.