I think it will be fine to try and treat this at home for now (even though we don't know the cause), but would recommend a visit to the vet if:
1) It doesn't get better in 2 weeks
2) It starts to get worse or have a lot of discharge.
Keeping the wound clean and keeping it covered are the key treatments for success in getting this wound to heal as soon as possible. For this, I recommend the following:
1) Wash/spray the area with just cool, clean water (no soap or other antiseptics) for at least 2 minutes at a time. Do this once a day.
2) Bandage the wound with gauze and apply an over-the-counter antibacterial cream (i.e Neosporin) to the wound. Here is a link to a good demonstration of a head bandage:
https://www.youtube.com/watch?v=K3c-SXhUSBYKeeping it covered and keeping Indiana from pulling it off will be the most difficult part of the treatment. You can help keep the bandage on by placing a good (really sticky) tape above and below the bandage.
NOTE: Be careful not to bandage too tight or it can cut-off the circulation. If you notice that the neck/chin area start to swell, then remove the bandage and leave it off until the swelling goes down.
These types of lesions tend to take a while to heal, so don't be surprised if its not completely healed until a month later. Once the skin looks normal you can stop the cleaning/bandaging.
I hope that helps and Indiana gets better soon.