It is hard to tell much from the photo as there are no distinct markings. The arm length in the second picture and the alteration of white and brown on the arms suggests O. briareus. Adult single arm length will be about 12" with a mantle slightly larger than a tangerine. O. hummelincki will have a slightly stiffer but smaller mantle with a single arm length of about 8". Both need a 55 gallon or larger tank preferably with sump and skimmer (I would not consider O. hummelincki without a skimmer but encourage a skimmer for all octopuses).
Definitely not a dwarf and will work well in your 125. I am not sure I see the ring you see. I do see a possible circle patch under the eye but that may just be angle and coloration. If you see a blue circle below the eye (ie not around the eye) then O. hummelincki is a definite possibility.
I just took another look and did see the blue so I think you are correct in you thinking. This definitely eliminates O. briareus and there is only one common eye spotted octopus in the area. However, I have had one animal from them that had a brown eye spot and was kind of a gold color much of the time. Monty was likely something that rode a current up from South America but we have not seen another This little guy is not the same unknown species but I wanted to mention that ID's are not always possible.
Okay I will most certainly do that! Do you think it would be best to find something to put him in inside the tank that is smaller or should I just let him have the whole tank to himself? I want to make sure he's eating
Ghost shrimp or very small fiddler crabs should be good. The problem will be finding it to feed if you are going to try freshwater shrimp (fiddlers will live in full saltwater - disable claws on males). If you can get an order of saltwater shrimp, it may be able to catch them on its own but monitor (I usually start with small fiddlers and try stick feeding the shore shrimp, The link is to the vendor I have used for years but local sourcing is cheaper for shrimp, Paul is probably cheaper for fiddlers). You can try eye sized pieces of table shrimp on a stick (once you locate its denning spot - starting with small pieces is important, food too large will be ignored). Bamboo skewers (look for the very thin ones until it can eat larger food) work well as feeding sticks but if you have the old nylon style, it works equally as well.