I am wondering if his tongue is still restricted MiauMau especially after hearing that breastfeeding didn't go well even after the snip.
There are several possibilities here. One is that the snip healed back together (totally possible, you cut the body and it tries to heal). If there is no after care done then this can happen, especially with a posterior tie, and I am willing to bet that you weren't told to stretch the site afterwards.
Another possibility is that the snip wasn't done deeply enough and the tie was never properly released (for a posterior tie).
Or, it is possible that an anterior tie can be snipped leaving a posterior tie in place.
helpful article here you may recognise some or many of the symptoms.
Has he got any other tongue tie related problems? Sleeping/digestive problems/high palate for example?
DS1 had undiagnosed posterior tongue tie and although he didn't vomit with solids, he had trouble with certain textures, interestingly avocado was one, which you also mentioned for your ds. He used to gag on stringy things like squash and he couldn't chew small stuff like peas which came out completely undigested at the other end.