As others have said, you will be fine. Another Portlander here. SW Flights from Portland ,particularly morning flights, almost always originate in Portland, meaning there will not be any through passengers on the plane when boarding begins. Math is your friend here. With B29 and B33, you have, at most, 90 people in front of you. Most likely, there will only be 80 or so, since the full 15 slots allocated to Business Select are rarely all taken on flights out of Portland. SW planes are configured with 175 seats - typically 58 groups of 3 seats (there are some exceptions to this, but I haven't seen the older configurations out of Portland for quite awhile). So, worst case - and highly unlikely - every person before you is traveling alone and they all take only aisle or window seats. 80 people would fill 40 of the 58 groups of 3 seats, leaving you with at least 18 groups (or about 9 rows) available for you and your son. For every two people traveling together, and taking a middle seat, another row becomes available for you. It is virtually impossible for you to NOT to be able to get 2 seats together. Should there be some very unusual circumstance where there are not 2 seats together, let the Flight Attendant know. SW is typically very good about asking people to shift their seats to accommodate families.
Steve