It does sound normal. Rabbits tend to rest a lot during the day, esp if they’re alone and nothing is going on. When they rest, they will poop a lot less than when they’re active.
It’s reasonable to think that it was the introduction of greens that caused the gas. In general rabbits can ve started on greens at around 3 months of age, but that’s not written in stone. Some buns need more time to develop a sturdy and diverse gut microbiome that can handle new food, so it seems wise to keep him on hay and pellets for a bit longer. Hay is the best thing for the bunny gut, it’s the perfect food for all the beneficial bacteria and yeasts that a bunny tummy is dependant on.
Nothing of what you’ve describe seems to warrant a vet visit. It’s of course always good to get a yearly vet check-up. In areas/countries where vaccination (against RVHD and/or myxomatosis) is recommended, the vaccinating vet will often do a basic exam.