I am presuming here that the purpose of your question is preparing for the naturalization interview? If so he needs an "government issued ID" for the interview and a valid passport. He should be having both of these. If he went outside the US, then passport stamps will show the time present in the US. Regarding income, there is no requirement to be working for obtaining naturalization. However he needs to show he is not living off public welfare. If you are providing for him, I suggest you claim him as a dependent and provide him with the tax transcripts that he can carry with him to show to the USCIS officer that on naturalization he is not likely to be a public liability because his living costs are met by his son.
