Actually, more is explained in the previous film, Friday the 13th part IV: The Final Chapter. Tommy Jarvis has a hobby of creating and customizing masks even before he meets Jason Voorhees. The first time that we actually see Tommy, he is wearing a mask of his own creation:
TOMMY'S MOTHER: That's a nice mask. You're getting terrific at making
TOMMY: Thanks. I just customized it.
The plan was to show how Tommy loved masks and how Tommy would be the next "Jason" picking up where Jason left off:
From an interview with Director Joseph Zito:
I wanted to make it so that Tommy could be Jason. I wanted to make it
so that Tommy could be the link into the future even if he's not
Jason. I wanted to open the door to other movies, but there wasn't a
strong interest by the studio to make other movies, however I did
leave it open just in case.
So...filmmakers were trying to move the franchise in a different direction with the ending of Part IV and V attempting to make Tommy Jarvis the killer.
The previous film in the series bore the subtitle The Final Chapter
and the producers of this installment intended to take the franchise
in a new direction, but fan outrage over the infamous twist ending led
to a more traditional approach with Jason Lives.
The ending of part IV: The Final Chapter left fans thinking that Tommy might be the new Jason. Filmmakers would do this again with the ending of Part V: A New Beginning leading fans to believe that Tommy would be the next killer and that killing Roy the ambulance driver threw him over the edge and he finally snapped and put the hockey mask on to become Jason. However, fans were extremely upset that Jason Voorhees was gone and did not want Tommy or any other "copycat" killer.
Tommy returns in Part VI and inadvertently raises Jason from the dead.