do we need a special power of attorney?

You don't need a special power of attorney to certify that your husband allows you to travel with your son.

I know that there is a rule: To prevent child trafficking, Republic Act 7610 (otherwise known as Special Protection of Children Against Child Abuse, Exploitation and Discrimination Act) requires unaccompanied minors below 18 years old who are traveling abroad to secure a travel clearance from the Department of Social Welfare and Development. A Travel Clearance is not required if the minor is traveling with both his/her parents. If, however, the minor is traveling with only one of his/her parents, the parent who will be left behind shall execute a duly-notarized Parental Travel Permit (in a form prescribed by DSWD) certifying that he/she is allowing the minor to travel with his/her spouse.

But in practice, the Parental Travel Permit is not required if the minor travels with mother.