He has to be celibate and never sin again?
Like I just replied to another poster, if he slips up and sins, he can go to confession, have his sins forgiven, and then receive Holy CDommunion. As far as remaining celibate, that would be the ideal, but if he doesn’t, then he cannon receive communion unless he asks forgiveness in the Sacrament of Penance.
Remarrying would eliminate him from doing that, because presumably he would not refrain from marital relations with his 2nd wife, and he could not make a good confession if he has no intentions of refraining from sinning again.