Foreign_Soul, why not turn this into a weekend away together? You could go with your partner to the other state and bring your son. That way the three of you are together. You, your son, and your partner can get a hotel room together. Then, the next morning, your partner goes to his best friend's wedding ceremony, which means you and your son can leave your hotel and go explore the city with some planned sight seeing activities.

Then, once your partner is finished with the wedding ceremony, can meet up with you and your son, and then decide if your partner will go to the wedding reception, or spend the rest of the weekend with you and your son sight seeing.

Make it a fun family trip. This is the only realistic solution I see for you, aside from barring your partner from attending his best friend's wedding or breaking up with your partner of 7 years b/c his friends are total jerks for not inviting you and your son as your partner's plus one.

Or, you stay home with your son, and plan for a friend, relative or one of your son's aids to stay with you over the weekend that your partner will be gone, to help you care for your son while your partner is away. And, have your partner call your son while he can, and work around the unknown-time factor since your partner's friends won't accommodate your partner with the wedding ceremony and wedding reception time or venue information, which is very odd.