Do you mean he has the gas and poops while with grandma, or when with you AFTER a day with grandma?
If the former, maybe grandmother is more concerend about gas and explosive poops than need be. If the former, I would wonder if maybe: baby is being overfed with the bottle (very very typical) and/or being fed something else than your milk. (Not as typical but it happens.)
5 ounces would be on the high side for a feeding at this age.
why not give grandmother suggestions for paced bottle feedign to slow the feedings down, and try more frequent feedigns of, say, 2-3 ounces at a time? See this for the current thinking on how a baby ‘should’ be bottle fed
http://www.llli.org/docs/00000000000...astfedbaby.pdf
You may need a different bottle as well. I have heard (but do not have personal experience) that the tommy tippes have to be turned almost upside down for milk to come out. This would put baby more flat on back for feeding which is not usually comfortable for babies and may be contributing to this problem. (Yes I know that is how all the pictures of bottle feeding baby is on back) but baby should instead be more upright as described in above linked tip sheet.
And try not to worry! Gas and explosive pooop may be temporarily upsetting for all involved, but they do not in any way represent a health problem that requires intervention.