5 months is a bit young to be at a dog park - especially if your puppy doesn't appreciate all the excitement that's normally abounding at those kinds of places.
It's not always about 'aggressive' energy... sometimes it's the over-excitement that puppies and dogs don't agree with.
Mastiffs can also be independent souls, and many grow up to not enjoy the company of non-family dogs, so Puca may fit that description already.
You might try finding a low-energy puppy group - maybe a trainer in the area runs some puppy-play-groups? He might find dogs his own age are easier to get along with, and not quite so scary.
If your dog is not enjoying the dog park... I would not take him there. That's a recipe for disaster if he decides to snap at the other dogs to keep them away - which he might (even happy ones, looking to play) if he has a very large personal space bubble that he'd like to maintain.
Another option - when you go to the dog park, just sit outside the entrance at a distance Puca finds comfortable... the other dogs should be on-leash at that point, and just passing by. Puca will hopefully not feel pressured to interact, but you can reinforce good behaviors (i.e. calm curiosity). Once Puca is comfortable outside the park, you can start to venture in. Let Puca take the lead, so he's not feeling pressured (which can lead to stress and anxiety). If he wants to go home... I'd go home, or maybe visit a park on leash without other dogs in his face, and just enjoy each other's company.
I wrote that before your last post... so... yeah, could be a picnic table thing... or just crazy smells all over the place that he's finding a bit overwhelming?
