This is what is says:
"No Title of Nobility shall be granted by the United States: And no Person holding any Office of Profit or Trust under them, shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince, or foreign State."
ARTICLE I, SECTION 9, CLAUSE 8
So the question would be can it be proven that any business he owns is benefited by him being POTUS.
The funny thing is, in the early years of the USA, congress et all would meet for about 3 months then go back to their days jobs the rest of the time