I have been in your situation so many times I could not count. My spouse has had PTSD for over a decade untreated until recently. We are currently in this phase as well.
The isolation can be very confusing, if there are occasional moments where the sufferer is loving, or overly loving for a tiny window of time and then reverts back to isolation.
The main thing that has kept me sane ( or mostly sane) through the isolation is not to take it personal. It's not your fault. The sufferer is not doing it to punish you, or ignore you because of anything you have or have not done. The behavior is about them not coping well. Isolation is common.
My spouse is unable to handle crowded shopping trips, so I have stopped getting upset that I must beg for his company, and allowed him to set the pace for what he can/cannot handle. He also can isolate emotionally and physically for periods of a few hours to a few weeks.
In your case, your young child will not understand daddy's roller coaster of emotions. This is where you will have to be stronger at setting boundaries so that the little one isn't taking the isolation personally as well. Daddy needing to disconnect isn't the child's fault.
What I have found works best is if I am consistent with my behavior, regardless of how my spouse is behaving. What I mean is, I always make dinner. My spouse may be having a bad day, or even a bad couple weeks, I will keep things consistent and make dinner for the family. My husband can choose to eat or not, but I always offer. I also do not take personal his choice to eat or not eat, but I used to get very upset.
Maintaining perspective that we are not to blame for the behavior we are experiencing. What I mean is, that the isolation and disconnect is a survival skill for the sufferer, not meant to punish the loved ones.
My spouse has made clear to me that he will isolate as a means to control the rage, because he does not want to risk exploding on me. To him, he is showing love by removing himself from the equation.
Many times, he isolates because he is struggling with recurring memories of the trauma. He has horrible nightmares and the traumatic events consume his thoughts and he can't cope with it, so he withdraws into himself. Never has it been directed at me. As I stated, it is a survival skill for him.
The few times my spouse has opened up about his feelings, he has expressed tremendous guilt over the rage and the pain he is acutely aware it has caused me and our daughter. I spent years taking it all personally.
Maintaining consistency in your behavior will create security for your child during some of the more challenging symptoms of PTSD. When they get older, it will be easier to explain why Daddy needs quiet time.
How you respond is more important than whether he eats or not.