Some clubs love players participating in ODP, some clubs ignore it, and some clubs/teams work against it. Some HS coaches are laissez-faire concerning concurrent play [either club or ODP] and some are deeply against it. You need to ask your coach [and in club, perhaps your DOC].
Conflicts between ODP and club have been minimal in our experience just as conflicts between ODP and HS have been minimal [to non-existent]. If your son eventually makes the region or national pool or team, your son will probably have conflicts; how these are handled depends entirely on your specific club or HS coach.
While almost all coaches at the club and HS level are very receptive to players attempting to increase their skills and are willing to work through conflicts, there are a few that are really against any potential conflict. Talk to your club or HS coach and ask about conflicts, penalties for missing practices and games, and how the coach feels personally about concurrent seasons of play.
After that, you can prioritize and help your son make the best decision based on his talents and goals.