Feeding him an Issur:[1]
Although a person who is halachically classified as shoteh (legally incompetent due to mental incapacity) is exempt from the obligations of Torah and mitzvot, nevertheless it remains prohibited for others to actively cause such a person to transgress a Torah prohibition. [Accordingly, it is forbidden to feed a mentally incapacitated person non-kosher food. Even in a case where such food is required for health reasons, as long as it does not rise to the level of pikuach nefesh, one may not feed him Biblically forbidden food directly. Rather, it may be given only through a gentile, as will be explained in Halachah 8f.[2] Regarding rabbinically forbidden foods, see Halacha 8c.]
[1] See Admur 266:10 “Nevertheless, when one gives it to him—or even to a cheresh or a shoteh—it must be given to them only after they have already lifted their feet to walk, and it must be taken from them when they wish to stop, in order that they not perform an act of lifting and placing (akira and hanacha), as explained above. For even though one is not commanded regarding their resting [i.e., cessation of labor], nevertheless, one is certainly warned by the Torah not to feed them a prohibited item directly, as will be explained in siman 343. And this, that one gives it to them in such a manner, is permissible, even though…”; Michaber 266:6; Tosafus, Piskei Riaz, Ramban, Rashba on Shabbos 153b; Rabbeinu Yerucham Nesiv 18:3; Kneses Hagedola Y.D. 157:56; Chasam Sofer O.C. 83; P”M Pesicha Koleles 2:1; 266 M”Z 4; Pesach Hadvir 343:7; Imrei Bina Shabbos 9; Amudei Eish 4:6; Kaf Hachaim 343:8 and 21; Toras Hakatan 16:4-11
Other Opinions: Some Poskim rule that there is no Issur of feeding a Shoteh non-Kosher food or other Issurim. [Terumas Hadeshen 62, brought in Chasam Sofer ibid; Shivas Tziyon O.C. 4; See Piskeiy Teshuvos ibid footnote 121]
[2] See M”B 343:5; Piskeiy Teshuvos ibid footnote 124