First and foremost, you need to talk to a lawyer. This is a case not a question and I’m not allowed to provide the legal counsel you need to deal with it right now.
Here’s what I can offer in the way of general guidance–as opposed to legal advice. The starting point is that ABSOLUTELY 1000% YES–you WOULD be exposing your organization to potential liability for wrongful dismissal and disability discrimination. For one thing, I’m not sure I understand your reasons for termination. Are you saying the employee has been dishonest? If so, you need to thoroughly document this with specific incidents. What questions did you ask? What answers did he give? Have you consistently treated all employees in similar situations for similar misconduct?
If grounds for termination is lack of physical capability to do the job, you need to perform a thorough medical evaluation. Did you perform one when you hired him? If so, what has changed since then? If not, why DIDN’T you do such an evaluation? There are a lot of red flags here.
Bottom Line: I strongly suggest you refrain from making any decisions in regard to termination until you talk to and get advice from a lawyer. These are shark infested waters you’re wading into. Good luck and feel free to contact me at glennd@bongarde.com