I brought home a poodle terrier mix puppy when he was 1.5 month. He has long soft fur and very fluffy like a bear. One day, I decided to trim his fur because I though he may looks like "boo" the dog. It was a disaster. It has been almost 2 months since I trimmed his fur. His fur has grown a little bit longer but he's not fluffy anymore. What should I do?

Not related to his fur, but why was he removed from mom so young?
Secondly, he is still very young, that puppy fur is temporary, he will eventually shed the puppy fur and gain his adult coat gradually as he ages.
If you are truly unhappy with the trim, take him to a professional groomer and have him trimmed.
If he is a dog that will need regular grooming it is a good idea to let him start adjusting to going to the groomers now.
If you are going to continue to trim him at home, please be careful and don't use scissors. We have all sorts of injuries come in after well meaning owners have trimmed their pets at home, on of the worst being a tongue literally split in two.