Tips/Resources for Kindergarten Adjustment?

Our son had his first two days of kindergarten today--a big milestone!  Overall, he did pretty well it seemed after settling in--but definitely had quite a hard time with the separation.  We're also eager to encourage him to socialize at recess, etc. From the recaps we've heard he seems to be playing solo & watching more thus far (he has a tendency to like to size up for a situation for awhile...once he's comfortable in a situation he can be the life of the party).

Would love to get ideas, tips and reading recommendations for helping him adjust.  Most of what I find on web searches talks about how to prep over the summer for the transition for kindergarten--well, not so helpful at this point!  Virtually nothing I've found talks about how to navigate bumps in the early day of the transition. Anything I should read?

A bit more background on our boy--he's on the young side for his class, just made the cutoff of turning 5 by 9/1.  But we didn't hesitate on sending him b/c he did great in two full years of pre-school, his teachers strongly agreed that he was very ready.  He's advanced on verbal and other academic stuff; we definitely were more concerned about the social piece--especially as he's an only child.  But definitely by the 2nd year of pre-school he was flourishing socially as well.

Our community is one of the few around us that has full day kindergarten...1 complaint we've been hearing thus far is "it's too long".

I'm confident he will do well--and feel he is ready to do so.  But we just are looking for ideas & resources re how to best support him.  Any and all ideas welcomed!