Well the fresh air and sunlight can't particularily hurt him as long as you are really careful about supervising his outtings. If he seems at all stressed by it, then of course, you'd want to adjust things but they don't stress nearly as easily as ball pythons. Do make sure whatever grass he's on hasn't been treated for weeds, had fertilizer put on it or is near or under trees that have been sprayed for anything.

We were so lucky when we had the big female boa that we had a completely screened in porch. She could go out there and exercise, get fresh air and indirect sunlight, watch the squirrels in the yard but was under control (she was over 6 feet so loose in the yard wasn't a great idea due to her size and our neighbours freaking out).