He is on a slippery slope if he wants to criticize the President. UCMJ Art 88.
https://www.thebalancecareers.com/punitive-articles-of-the-ucmj-3356854
The exact words of the Uniform Code of Military Justice Article 88 - Contempt Toward Public Officials states: “Any commissioned officer who uses contemptuous words against the President, the Vice President, Congress, the Secretary of Defense, the Secretary of a military department, the Secretary of Transportation, or the Governor or legislature of any State, Territory, Commonwealth, or possession in which he is on duty or present shall be punished as a court-martial may direct.”
Is he not retired? I fail to see what slippery slope he is on.
"The main reason for this regulation is to keep military members who have access to major weapons of war to ever get involved in politics. Once they are retired or resigned their commission and a civilian citizen, they may partake in such political arguments in both written or spoken word. With the advent of social media is can be a slippery slope for military members to discuss such matters and could even be subject to UCMJ violations. That is why you will find military members refrain from that activity or have incognito social media accounts."