If, during your declare attackers step, Flameborn Hellion is tapped, is affected by a spell or ability that says it can't attack, or is affected by "summoning sickness," then it doesn't attack. If there's a cost associated with having Flameborn Hellion attack, you aren't forced to pay that cost, so it doesn't have to attack in that case either.

If there are multiple combat phases in a turn, Flameborn Hellion must attack only in the first one in which it's able to.