You have been given great advice above. However, I wanted to clarify some things to avoid confusion.
It depends how much time and effort you are willing to put in to your bird, really. [...] Budgies love social interaction, they love to sing and chatter - they are not solitary animals at all. If you don't think you can give him all the attention he needs i would get him another bird BUT you will lose him as your best bud. They will eat together, feed each other, fly together, talk to each other. There's no need for human companionship when they have that birdy bond.
I wouldn't dream of getting another bird for Joey because my daughter would lose her best friend. But, not everyone is 12 with time on their hands and the inclination to spend hours teaching him to speak and play peek-a-boo!
This is true,
however, it is very important to take into account the temperament of the bird. Budgies are social animals and have a "flock" mentality. For some birds, this can extend to their "human" flock, and they get the
same type of fulfillment being with humans as with birds. This is the case with my bird and with several members' birds; they identify with their human family and in some cases may respond with aggression when introduced to new birds. However, some budgies especially are not suited to being solo birds
even with extensive human interaction and devotion. Some just do not have an extroverted enough personality to extend their "flock" mentality outwards to humans, or, even if they do recognize their humans as part of their flock or family, they still feel isolated without a bird friend. It is, of course, more likely that budgies have a familial bond with humans if their owners spend considerable time interacting with them, as mentioned above. However, this alone doesn't guarantee that they will be well-suited to being solo birds. As you get to know your bird (3 months is a very short period of time) you will be able to identify his personality a bit better.
For your original question: only get another bird if you want another bird. In the event the two birds don't get along, of which is a 50% chance (though there may be ways to mitigate this), you'll need a separate cage.
Along these same lines, this poster is also correct, since having two birds does not guarantee they will have compatible personalities. However, you also should not get a bird just because you want another bird and should first consider if you are prepared to get another bird and if you have the space and resources to care for an additional bird including, as Vrabec mentioned, in a separate cage. In the situation that you do not want another bird and do not have space or resources to care for a new bird, but your budgie appears to have difficulty bonding with humans and would prefer another budgies' company, in this case, it may be best to find a home for your little one where they can live with other budgies. This doesn't seem to be the case here yet so that's why it most likely wasn't mentioned, but in case other members with similar issues are reading this.
As asked above, how much time do you spend with him? What kinds of activities do you include him in?