I only have a minute b/f heading out & wanted to give you some of my thoughts re; child custody.
I went through the court system w/ my S15 when his dad attempted to keep him after his 5 week stay in Hawaii visiting him.
It doesn't always turn nasty, though it is an adversarial process in nature, so does "pit" you against ea other to a degree.
When I read my XH's version of our respective parenting roles I was floored. I drove to LAX to work the Red Eye Flts back & forth for years so I could be at home during the day to stay w/S. Since my payroll just reflected I was paid for certain days, etc., it made it appear I was gone all day & night & therefore he took care of S.
My point is, in retrospect I would have done many things differently.
The courts don't care if it's the mother or father & generally like to see the child remain in their routine/home.
Your H appears to have seen an attny & is doing what ever he can to establish primary cust.
I, o/c, am not an attny, so can't tell you what is best exactly, just what I might do:
-move back in the family home & begin setting your own new precedent. The courts look at the latest history mostly.
-start going to as many appts (Dr., etc) w/ D as possible & record everything everyday.
-If you don't move back in, take D with you to your place & make it for over the 1/2 mark (which is what it appears he's trying to do)
-Don't respond to anything he sends re;sched for D in writing-it will/might be used against you.
Unfortunately, this doesn't help the connection with your H & will bring up some pretty powerful feelings for both of you.
My former H & I are very close now & have been for many years. It wasn't until almost a year after the court stuff that we were able to get there though.