Fleetside side panel dented

[Samz]

My son has a '66 Fleetside (short bed) and there is a large dent on the right side in front of the right rear wheel. The dent is approx. 12"x12". The previous owner apparently tried to fix it likely with a slide hammer...it didn't work! I tried to pull it out, but every time I pull it out, it pops back in. I'm no auto body guy, but I do know how to do some basic bodywork and paint (I did my '60 C10 last year). Does anyone know how i can get that dent to stay in place? I've been researching "metal shrinking", but I've never done that. I'm not afraid to try that, I just want to make sure I have a good idea on what I need to do. I did try a slide hammer too, but when I pull the dent out, it'll pop back in once I let off the pressure on the slide hammer. I know the best thing to do is to replace that side panel, but my son's funds aren't allowing him to do that just yet. The truck is not a show truck, but of course we'd like to do the best we can. Thank you in advance.