Jen, I want to tell you not to stress out over this.
This is the part of teaching preschool and having a kid that I hated: those darn developmental milestone markers. People seem to feel they are holy writ; they aren't. They are a loose generalization of averages that get clumped into months which may or may not correspond to any individual kid.
Speaking as the mother of a kid who did everything (and I mean EVERYTHING) at least a month or more later than that damn chart, I made a choice not to let the chart affect how I perceived my son. I think you will need to do this too.
(Um. just because I'm wondering,what is a 2.8 year old break down to in actual months, b/c most people break it down as either months or year/month, but not decimals...)
From my perspective, it sounds like your son is pretty much on target for his age. I agree with educating yourself on the overall development of the young child. Parallel play and playing alone with what interests them is common. "Checking out" could mean anything: sensory overload; thinking about what's going on/being said--which is a lot of work for some kids--or their mind is on something else. Don't we all have the right to have our own thoughts? We all know adults who 'check out' at times, right?
If it's any consolation, my son did this some times at preschool--- wasn't really interested in playing with other kids, even at nearly four, didn't participate much in group activities.... so the preschool teachers helped him and taught him how to go about being in the group. His first year of preschool he spent most of his time observing or involved in his own pursuits; the second year, he was doing what the other kids did (singing/music and movement) at circle times, playing with friends, contributing to conversations... it just took him more time than it took some kids.
Each child has areas of growth and those areas where they need to develop and they can't progress in ALL areas at once. Don't compare your son to other kids (first rule of parenting) and just try to relax and see *who* he is, what he is good at all on his own. Focus on that. Play with him when you have time, and don't let your worry for him supersede your enjoyment of him. Kids pick up on our worries and fears, so go forward carefully and do not talk about this in front of him if at all possible. Good luck.
Ditto everything Doris Day said.