If he did istibra, which is the process of ensuring that there is no urine left in the urethra, or if he knew that the urethra had no urine left because he waited for some time after urination or because of any other reason, then the prayer is valid and he should not pay attention to this doubt. If this was not the case, and he doubts whether the liquid was urine or not, then he should treat it as urine, consider the wudhu and the prayer as void, and purify the body and clothes that the liquid came into contact with.
