Must a convicted DUI felon disclose this conviction on a job or college application and is he likely to have difficulty finding a good job if he does disclose it?

You must disclose it if asked about your criminal history. If you don't, it'll come up in a background check and you'll be seen as a liar. If you tell the truth, you have a much better chance of success. No one's perfect and most employers and schools will recognize that.