R.J.
It's CYA for all of you.
Just like a grandparent, neighbor, etc... Your STBXH has no legal rights to your son until custody papers are drawn.
- Med treatment
- Travel
- Insurance
- kidnapping (claims of)
- biodad
Are all potential HUGE problems unless you write out 2 forms. 1) your permission to seek and obtain medical treatment in your absence from x date to y date... And 2 a document saying you give your permission for son to be in STBXH care from x date to y date.
From working in healthcare, stating the relationship (stepfather Jon Smith) is helpful... But the DATES are even more helpful. States vary, but these notes are invalid without them, and are only good for 24 hours to 90 days in most places. (24 if you just write today's date /signed on)
Whenever I've left my son with my parents for longer than a weekend -or even a weekend if I'm distant or out of reach, hiking say- I always leave the same 2 notes. And this was BEFORE I filed for divorce. Now that courts are involved its even more important. And that relationship isnt in the process of severing.
Don't take it as an insult (the idea you might call the police and report custodial interference/kidnapping). You might NEVER do such a thing, but others would. And well meaning friends/family MIGHT STILL on their own thinking they're helping you out by keeping you out of it. (Ahem. Including your son, of he got really mad.).
It's just to protect what BOTH of you want. Which is the time with SS&SD