First of all, he is home now. He's El Salvadoran, remember?
Second, maybe if Trump asked Bukele to release Killmore and allow him to illegally enter the U.S. again, he would. Pam Bondi said she'd send a plane if Bukele does that.
But the USSC did not direct Trump to ask Bukele anything. Lower Court Judge Xinis may have thought that she was ordering Trump to do so, but the Supremes did not uphold that.
Sure, he "could" do something. But what you suggest would not be reasonable. He "could" send in Navy SEALS to pull Abrego Garcia out of prison and take him to the U.S. He "could" order a mansion built for Abrego Garcia to live in luxury and comfort free for life as a way to incentivise him not to carry out any more gang assassinations. But none of that would be reasonable.
Leaving it to El Salvador to deal with its citizen is very reasonable.