If he has all his shots up to date, he'll probably be just fine. Possums - opossums, officially - are one of the few marsupials outside of Australia! Poor little 'possum. RIP.

Bllllech, Simba! and worrying your mama like that! He was just carrying it, not eating it? Do you think it was already dead, or that he killed it? Check Simba carefully for any scratch or bite marks. If he has any, just keep them clean so they don't get infected.