I think I know why he entered the 16s I think its because he has won the Orange Bowl 12s and 14s and now he wants to win the 16s and eventually the 18s, I'm not sure if this has been done before but perhaps he wants to be the first to do it. He has said the Orange Bowl is his favorite tournament.
I'm sorry to tell that Tomic will not be the first player to win Orange Bowl in every age division.
One girl and one boy has done this before.
Mary Joe Fernandez
is the only player to have won four age divisions in four consecutive years the 12s in 1982 at age 11, the 14's in 1983, the 16's in 1984 and the 18's in 1985.
holds the distinction of the best Orange Bowl boys' record. He reached the 12s final in 1967 at age 11, won the division in 1968, won the 14s in 1969, won the 16s in 1970, then in the 18s was a semifinalist in 1971, a quarterfinal loser to Borg in 1972, and winner in 1973 and 1974.
Source to this is following very interesting article