Son charged as felon at 18?

My sons ex girlfriend showed up at his house bruised and beaten by present boyfriend. My son was 18 and took it into his own hands to "take care" of her abuser. He beat the young man and as further humiliation asked if he had any money, the boy had 20 dollars which he gave to my son. This was first severe offense, only 1 minor marijuana possession was on record. 2nd degree ault and robbery charges were filed and my son received a disposition of "Nolle Prosequi" for the ault charges and suspended 2 year jail sentence for robbery charge. This roberry charge was considered a felony (even though the amount was 20 dollars) and now he is 24 and had a hard time finding a job and rental agents do not want to rent to a "felon". Court record search does not list amount of robbery, just states it is felony under 500. Is there any action that can be taken? I have been asking for 5 years. Sent letters to Senators and Governors with no response.