There is not much such games because the vast majority of incest games where we have MC and his mom however also have other characters besides MC himself and his mother but here are games which I know where we have only MC and his mom.
Debt - RPGM, game completed.
MAMA - game is very poor in my opinion, firstly translation is very poor (MTL) and on top of that game is censored like most Japanese games unfortunately.
Itsumama 2 ,
Itsumama 3 ,
Musumama ,
Musumama 3 - Japanese games so unfortunately censored.
True Bond - here we have not real incest because MC is adopted and so far we have only his foster mom with her husband and MC in game as the only characters.
Measuring My Cum - game unfortunately abandoned.
Staying With Aunt Katie - RPGM, game completed , here we don't have MC mother but his aunt, but you might want to try this game too because MC and his aunt are the only characters in this game.