Hi, thanks for the advice!
As I haven’t noticed much (if any) growth, I’ll give it about a week or so to see if it does grow or shrink. And if it stays the same or increases in size I will take him to the vet. Like you said, it’s very difficult to tell what it it just by a photo or without a biopsy, but I did do some googling and it does look similar in appearance to a Histiocytoma. Even so, he will most likely go to the vet.

Thanks for the advice!