Sorry to hear about your Dad. What's his BMI do you know? I am also overweight and BMI 34 I think which is not good, 109 kg and should be 73kg.
Here are some things that you may try BUT you can ask your doc if these things are advisable in your dads case.
First thing in the morning to drink a large glass of water so that it creates a weight in the stomach and then to step off maybe one or two steps and jolt the hernia back into place on landing, some people say this really works.
Eat small meals every few hours, no big meals and none after 7 or 8pm, 3hrs before going to sleep.
Walking briskly as long as possible, say 30 mins daily or more.
Massage the solar plexus area downwards softly and towards the left (stomach) see how he feels.
Practise abdominal breathing exercises, i.e. filling the lower belly everything under the breastbone and when breathing out slightly tense the abs whilst belly goes in, this can make him relax deeply as well and allow the diaphragm to sink down hopefully.
Don't laugh - get him to blow up balloons, the pressure needed to blow up the balloon will make his diaphragm work apparently, every few days change balloons as they get too elastic and easy to blow.
Get him to have good posture when walking, and sitting.
If possibly when he sits to defacate tell him to squat like the chinese do, it is awkward but it takes pressure off the hernia when going to the toilet, strain pushes the hernia up, he'll have to put his feet on the toilet. but worth trying for a while.
Prayer could help of course and even talking to his body like I love you and placing his hands on the solar plexus and asking the hernia to go down and softly massaging down, yeah I know not very practical but I think it's worth a go as well along with the other things I mean.
I would say CUT down on food intake, let that stomach shrink and complain, i.e. let it mumble and grumble and thats when to feed it again and not feed it when you feel like it's time to eat or feel bored or watching the game on TV or cause my wife made food or etc, but when your stomach complains and growls ... these are the things I am trying now, Good luck for your dad, tell him he's not the only one, others are in the same bloody boat as well, I'm not a great christian at all but I'll pray for him to be healed - let him know that, I mean that other people are sending good vibes I mean.
Seems though he's going to need an operation, in China I saw a diagram in a hospital of one operation and it seem's that the stomach is squeezed with a kind of band near the hernia area which makes sense, BUT if one doesn't eat and the stomach SHRINKS naturally surely that would either do the job or greatly reduce the problem?
Hope these help if I think of anything else I'll let you know.