First if all, well done for taking responsibility for the pup.
How long had your friend had the pup before passing away? If the pup is three months old now and you've had him a month the pup must have been pretty young. Pups go through what's known as a fear stage between 8-12 weeks of age; scary things that happen to them then can have much more impact than they would in an older or younger dog.
I would therefore think that the pup is not exactly sad, but fearful, and this will take time and patience to overcome. He's not just going to 'cheer up' overnight.
Do you know where the pup come from? Does he have pedigree papers? If so you could try contacting his breeder for advice. If you don't know or are unable to find out, ask your vet to recommend a good local behaviourist - not just a trainer - who can help.
Then, please get him physically checked by a vet to make sure he is in good health and not suffering any pain or discomfort. Are there any documents recording his vaccination status? If not, please ask what the vet recommends regarding this as they may be able to test to see if he has been vaccinated or not before acting accordingly.
There are some good resources here: fearful dogs.com (sorry, am using phone so can't get a clickable link). Also look for the websites of those organisations which rehabilitate street and feral dogs as they have tried and tested methods of helping dogs overcome their fears.
If you can, please give him a space in your home - a crate or indoor kennel - so he can make a 'den' to where he can retreat if the outside world gets too much, and where he can relax and eat.
You won't be able to force him to be 'crazy and fun loving' until you've helped him to come to terms with life and form a bond with you and trust you. At first this will involve waiting for him to take the initiative, and time, lots of time.