By RULE, not a TD. By rule 9-2-1-a-1-e, this is an unsportsmanlike conduct foul that occurs during a live ball, thus, by the penalty statement, it is to be penalized as a live-ball foul, according to the Basic Spot and the 3 & 1 principle. The basic spot would be the end of the run, which is the goal line. The foul is behind the Basic Spot, thus, the penalty is enforced from the spot of the foul. Repeat the down (or first down, if the penalty leaves the ball beyond the line-to-gain). The game clock will start on the snap (due to the apparent score). Play clock at 25 and starts on the referee's signal.
Philosophically, if you can rule the ball as having broken the plane of the goal line before he began the leaping action, then allow the TD and penalize as a dead-ball UNS (on the Try or the succeeding kickoff, offended team's option). Make this clearly as happening in the field of play, to be a live-ball foul.
Either way, tally this as a UNS to this player. If he gets a second UNS (or, if this is his second), he is disqualified from further participation in that contest.
Robert