Salam Alaikum, if a person (Shia Muslim) does not shower for a long time (weeks or months) and is generally unclean, is he considered najis and can he pray? if he is unclean but does wudhu for salah, is he still najis? Thanks

If you mean that uncleanliness is not going for bath then there is no issue he may do wudhu and pray but still remember cleanliness is part of faith and highly recommended
But if you mean najasat which causes for ghusle wajib then he has to do the islamic bath which called ghusl if Ind can not do fir reason like unavailability of water or water is harmful to him or time is short for prayer then he has to do the tayammum but he can not pray just with wudhu without ghusl or tayammum if one is in the state of najasat