Official Ruling
No, it can't attack. A new attack declaration can't be made during an attack. In this case, BT16-077 [Dinobeemon] is already attacking using its [When Digivolving] effect. This card's [On Play] [When Digivolving] effect can be activated before the counter timing, but because you can't declare another attack during an attack, you won't be able to attack using this effect, even if you activate it.
Source: BT20-016 Paildramon