The ex-Prez's son was at the Republican National Convention Monday and he talked to CNN about the immediate aftermath of the shooting -- when Thomas Matthew Crooks opened fire on Trump but missed ... only to be shot and killed himself almost immediately after.

Here, Don Jr. explains what was happening from his end of things -- saying he tried getting a hold of his father directly ... but couldn't get through at all for about an hour and a half.

DTJ says his father was in a lockdown environment at the hospital he was treated at -- but when he was finally able to get in touch with his old man ... Don Jr. says Trump was surprisingly jovial about the whole thing, joking about the fact he'd been shot apparently.

Don says Trump also showed great resolve on the phone ... which echoed how he reacted right after getting shot -- namely, pumping his fist in the air and displaying defiance. Don Jr. says he called his dad a badass ... and he even touched on how his RNC speech has changed since the assassination attempt. According to him, DT is gonna try to unite the U.S.

It was a fairly friendly interview with CNN ... although, the same can't quite be said about his interaction with MSNBC -- who also tried interviewing him on the floor, but things went south.