Some cats don't start to thicken up until they are 5 or 6 years old and others never will. Some are destined to be thin because both of their parents were. If you were able to see the parents, you can guage his size a little better based on that.
Keep in mind that being thin and active is more healthy for your cat than being thick and sedentary.
You might want to talk with the vet about getting the cat started on a kitten diet. This usually has higher calories and might help to put some weight on your cat.
Let me know if you still have specific questions. I would be happy to tell you what I know.