[Ali Imran]

If he wants to do it ..no one is stopping him
Although Ishmael was circumcised, God clarified that the covenant promises would be fulfilled through Isaac, Abraham's son with Sarah, who was yet to be born.

So you moslems follow it not because God made a promise with you...lol
It should be optional for the moslems lol

I will establish my covenant as an everlasting covenant between me and you and your descendants after you for the generations to come, to be your God and the God of your descendants after you. - Genesis 17

So, does the covenant include the Ishmaelites?