Sparks, glad to hear he's having 'fun', as compared to your reading of his attitude the prior week. We've had the talk before about not reading too much into one game or series of games, and how you've got to be the stable foundation that keeps the highs and lows balanced.
Sophomore year is tough for talented kids, because you have the "Var vs. JV" issues. Many times kids or their Dads beg to come up, and they discover either that the sit or they get humbled. Nothing wrong with just staying with the younger guys and having fun and learning. It's the junior year when they need to shine in order to get some attention from the college coaches. Also, your failings kinda disappear.
By contrast, our HS varsity has played two games and juniors with college aspirations (and talent) have started each game. Both started out well but stumbled - a hit and an walk or HBP or two then a seeing-eye grounder, and all of a sudden they've been yanked and the reliever couldn't stem the bleeding and ... BOOM, they're both looking at ERA's over 20.00 available for all to see on MaxPreps. Even worse, the coach has lost confidence and they won't have too many opportunities to lower those ERA's.
I agree with the others that notwithstanding the lack of advance notice your boy should look at the sudden chance to pitch as an opportunity, not as a conspiracy. It happens all the time when a pitcher gets hurt... or the star of a Broadway show twists an ankle and the understudy becomes a star. As far as the suggestions by others that your boy 'suck it up' and put his past troubles behind him, well, I've been blessed with a solid family and enough talent to get along, so I can't say I've walked in his shoes or yours. But I hope you freakin' appreciate - maybe via a PM - that TG643 has apparently lived that kinda life and moved on and is willing to open up to help give you some important guidance. Listen to him.
Your boy has had six years of a relatively stable situation -- I hope you've been able to shield him from much of the legal drama surrounding his unstable mother. Six years is a pretty good run and it hopefully has eradicated most of what may have haunted him. He's got to choose what goals he wants to shoot for and not accept excuses for getting significantly sidetracked. (Minor derailments are to be expected with teenagers.) So, when coaches do jerky things - and it's a given that they either will do so or will seem to do so (which is the same thing from your point of view), the important thing is to decide that you are not going to give a jerk the satisfaction of getting under your skin and throwing you off your game.
I've been subject to criticism in my life (even here!) and will internalize it to a greater or lesser degree depending upon my respect for the criticizer. But there are about a half-dozen people in my life who - if they tell me I'm screwing up - will get my immediate attention because I trust their judgment and know they have no agenda other than my welfare. And there are a great many more folks whose opinions mean little to me and I don't try to disabuse them of critical opinions of me, but rather just shrug and smile wanly and go about my business. Sometimes the best response is, "Whatever, dude"... and then walk away.
Your boy needs to do the same thing - decide those whom he's going to let get him upset. If the coaches whipsaw him back and forth as to his role on the two teams, the best 'revenge' is not to get upset but to go out and do the best he can and come back into the dugout with a look that says, "Okay, that was a fun challenge; what do you have next for me?" And the best 'revenge' for a bad early life situation is to do what TG and countless others have done - move on and not let those circumstances either pull you down or make you so bitter that your future relationships become impaired.