My heart aches for you, and so do my jaws, from laughing. Sympathetically, of course. Thank God this doesn't happen every day. The only solution I can think of to help contain the mess is to tear a terrycloth towel into strips just longer than his waistline and wrap it in a single layer, extending a couple of inches into the top of his diaper, and extending a couple of inches up under his onesie. It may absorb enough of the moisture from escaping poop to slow the upward flow.
I have a baby-vomit story that may give you a giggle. The Night of the Bean. My daughter was about 4 months old when we spent a night, on short order, at the home of some friends of my husband. Sparsely furnished room, no horizontal surface suitable for changing a baby except the bed, and a bathroom way down the hall with no counter surface. (It was not my choice to stay there, but my ex was insistent). We had no spare baby clothes, and only a couple of extra diapers.
At dinner my ex (could have been me, but we'll just say this was his bright idea, too) fed my daughter ONE bean, which she liked. She had no solids yet in her diet, just breast milk. An hour or so later we were upstairs getting ready for bed, and she started squirming and fussing. I picked her up to comfort her, and she suddenly puked an impressive fountain right into my very long hair. Left shoulder, I think, as an opener.
So I switched her to my right shoulder, and she produced an even more impressive fountain. Hair, clothing, me (front and back), her, the rug. I carried her down the hall and mopped us off as well as I could with a single guest washcloth and hand towel, having to hold her in one bedraggled arm at a time. Took a long time, and neither of us got exactly clean, or dry.
Got back to the bedroom to clean the floor (ex-hubby, disgusted by the mess, was unhelpful during all this). I said I wanted to go home, he said no. I said I was going to go ask for more towels, he was aghast - what would his friends think? That I couldn't manage caring for a baby?
So I leaned down to set my daughter on the bed, and the moment she was horizontal, she let loose with another fountain, at least 6 inches into the air, and my hair. And the bedspread. And her already wet clothing. Back to the bathroom with its soggy washcloth and hand towel.
That was the final barf-up, thank goodness. My daughter was fine. My ex was having tantrums for the rest of the evening, but stubbornly refused to leave. I don't recall what, if anything, we said to our hosts in the morning, but they had a large, new stain on their old bedspread. At least we were all dry by then, and heading for home and fresh baby supplies. I'm so glad I find the story amusing now. At the time, not so much.