I own a house in Illinois and have a tenant who is living there. I am in the military but my wife is having serious health issues and needs to move back to Illinois since I am about to deploy to Iraq. The tenant's lease ends in May - is there a legal way to terminate the lease early so my wife can move back in?

I don't think there is anything you can do to force the existing tenant to move.

Why not offer to pay him some $ to get him to leave early/terminate the lease?

If there are grounds to evict the tenant under the lease, you can do that. But I suggest talking to the tenant to see if they are willing to vacate. Also, read the lease to see if there is a clause that either party can terminate the lease with notice, usually 30-90 days.