I would buy plastic diaper covers - like potty training kids use, the thin plastic underwear type pants - and put those over his diaper. At least that way the blow out will be contained. You may have to go up to a size 5 diaper, sometimes that is the problem. Make sure that the legs of diaper have the elastic flat against his leg, and that the waist is tight.
As far as food wise - p's and b's (pear, blueberries, any berries really, prunes, plum, etc.) make for runnier poops, as does any juice. BRAT makes for more solid poo - banana, rice, apple and toast. So adjust his diet as needed with that. Maybe some rice and banana or apple each morning?