Hi, I just adopted a st bernard puppy, I'm staying @ a tropical rain forest country, the 1st and 2nd days he seem happy, but 3rd days onwards he started to become moody, keep lying on the floor and doesn't wan to eat and drink... I m starting to worry about his health and he become skinny and skinnier... anything I can do to help him ?

My advice is too get him to a vet.

Get him checked out.

If you have only just adopted him it might take a few days for him to settle in and get more comfortable in his surroundings.

Being a heavily coated dog in a hot climate (I'm assuming) wouldn't help either, try and get him nice and cool an hour before and after meal times.

Get some puppy milk to splash in his water to encourage him to drink and if you can grab some pedilyte just to keep him hydrated

If he keeps up not eating for more than a few days then definitely time to go to the vets.
If you do not get him to drink today then take him in, dehydration can be very serious.