Hi everyone,  
we adopted a dog from a shelter two days ago. So far, things are going really well. We’re still working on building a bond and trust, but overall he’s a very sweet dog and already listens surprisingly well.  
The shelter did mention one issue, though. When he gets excited and wants to play, he can become quite wild, especially on large grassy areas or open fields.  
He loves running around, rolling in the grass, and generally having a great time. But there are two things I’m struggling with:  
1. After letting him run and play for a while, I call him back. Most of the time he comes back just fine, but sometimes he stops right in front of me, lowers his front legs and chest toward the ground, and starts barking at me. It seems playful, but I’m not sure what he’s trying to communicate.  
2. As soon as we enter a field, he gets much more excited. He runs away from me, then comes running back and jumps up at me, sometimes nipping at my forearms. He doesn’t bite hard and it doesn’t hurt, but after a while it gets to be a bit much. Once he’s in this “mode,” I find it very difficult to calm him down.  
I already have an appointment with a dog trainer, but unfortunately it’s not for another three weeks. In the meantime, I’d really appreciate any advice on how to handle this and how I can help him calm down when he gets overly excited.

It’s like he wants to tell me this is his area and I’m not allowed to enter but also it seems like he’s requesting me to play with him. 

Unfortunately, I don’t have a better video yet that really captures the behavior, but I’m hoping to get one soon.  
This video shows his worst behavior in this case. 

Newly adopted dog gets extremely excited on open fields – looking for advice