The bullying is an issue on its own but the other issue is vandalism, which if it keeps happening, might be time to invest in surveillance then contact the authorities.

As for your son, you're fighting a losing battle since the parents of the 2 kids probably reinforce that behaviour. Your best bet would be to have your son gain additional friends or find kids willing and capable of protecting him.

For the martial arts option, you might want to consider self-defense classes but more traditional martial arts classes are certainly a valid option. At the classes I helped teach (both self-defense and traditional martial arts), many of the students enrolled because they were being bullied or abused. I cant speak for how all classes operate but we always emphasized to avoid a fight and many of the combat techniques were geared toward this goal; do something to briefly stop the attack, then run Forrest, run. Sparring for 8-year olds should/would be done with a generous amount of padding and protection, so he doesn't get hurt or hurt his sparring partner, however, it's unrealistic if he ends up in a 2vs1 fight. I cant imagine any instructor allowing an 8-year to engage in heavy sparring, which is more realistic but not something an 8-year old should be doing.

The martial arts option is a long-term solution because your son won't learn a lot in the first few classes, anything he learns won't be natural for him to do and he may not have enough confidence to do them. Hopefully, the threat of him knowing martial arts is enough to deter the bullies. Finding other kids willing to help is the short-term solution. Perhaps you could talk with other parents in the area and create a parent meeting to discuss how these 2 kids should be dealt with since they're not only affecting your son, property and yourself.