When I do for others what they can and should do for themself...that is enabling.
In the case you put before us... My "opinion" (so take it or leave it)
A balance of what is best for the child along with a need for him to be at men's group would of had me say yes I will help but maybe let him know he needs to check his schedule better.
If it is and has been a boundary you have placed and used in the past...you did right to hold it firm. If it is a new boundary...communication of that fact and letting him know it won't be done again would be a support and help to him.