The deal with the waterpark/ice cream is he was not wanting to leave. He saw that other kids were still there and thinks that with throwing his fit he will get his way.
So he was already IN a tantrum when you offered the ice cream? You didn't state that before. Well of course he ramped it up when he got to the car and realized he was going home AND he missed out on ice cream to boot. He was now missing out on TWO things - not just one.
So lets focus on the leaving. Did you give him fair warning that you were going to leave at that time? Did you say, "OK, we need to get going soon. Let's do 3 more rides and head home. That was a fun one. Two more rides and we're going home. OK, this is the very last ride for the day. Boy we've had fun and maybe we'll come back again, but this is the LAST ride." He needs the fair warning for transition. Now, I'm not promising that this will work the very next time you try it, but if you do it EVERY time you leave someplace (adjusted for the situation, of course. 15, 10 and 5 minute countdown ex for the playground) he will start getting the picture of the transition and it will become easier.
He's used to his mom doing things at the spur of the moment, with no warning. He tantrums, she changes her mind. he gets his way. You have a more planned way of doing things, but if you don't' include him in this plan, then he will react the way he always does, trying to get his way. If you apply transition warnings consistently and it STILL doesn't help, then there is more than just a transitioning issue at work - need more details.
Well, my son potty trained 2 months before his 4th birthday. I never pushed. Made it available for him, but didn't push. One day he decided he was ready and in less than a month he went from full time pullups to full time jockeys (even at night). My friend's son just finished potty training at 3 1/2. She had tried earlier and failed. Tried again and failed, and then one day he just decided to start using the potty consistently. Some kids just take longer. Yes, it's a real PITA due to the daycare issue, but at "almost 4" (sorry, for some reason I thought he was closer to 5) this is still not drastically delayed. Of course, the diaper ppl didn't make the process any easier with the "super absorbent stay dry liners"! I laughed so hard when one of the pull up companies FINALLY came out with the "feels wet" design - Major DUH!!!!!!!!!!!!!!!!!!!!!!!!!!
Anyway, do you see how I'm picking apart every little detail of what went on? This is a skill that you and husband are going to have to learn. "The devil" is in the details, and you have to pick apart the details of each situation, what may be going through his head, the way he is reacting to each step, and how you are both expressing yourselves and your intentions. It's not easy at first, but it does get easier.
And by the way see if you can get him back into that speech and behavior therapy. He's with you now so biomom should not be able to pull him out. IF she gets any visitation make sure it revolves around the therapy sessions so she has no responsibility to it. If you (you AND husband) are going to have any success at parenting this child, HIS needs MUST come first - not "keeping the peace"