I have a problem, and need HELP... Ever since I left my dog with my ex *he was my boyfriend at the time* for two weeks over a year (closer to two) ago he came back a different dog. Now, almost 2years later he has slowly become more and more aggressive towards people. Very few people can come into my home, or even approach him to pet him. What can I do to fix this? I have been trying to socialize him, but he lashes out when anyone but my close friends come near him. Heck, some people can't even walk by my house. I want to get him fixed, but cannot afford it I barely make enough to pay my bills and rent only work three shifts a week and those are never more than 4.5 hours a shift.

There are low cost programs for spay/neuter in place in most areas in the US.
Are you in the US? Call the local Humane Society, the shelter, local vet clinics and ask about programs in your area. Some rescue groups will provide assistance with this, it will take some work but you will have to seek them out.
IF and only IF you are comfortable telling us what state you are in, perhaps we could help you with that search or a member may have some personal knowledge of groups that aid with types of things.
Other than that there is Care credit, a charge card specifically for medical purposes, or if you can find a vet that will take a payment plan (many will not) but you may be surprised.
Did your ex give him back because of this aggression?

Neutering will probably help some, but a four year old male is at his most dominant. This is when he has been bred to be the protector and breeder, if not the lead dog in the group. Positive reinforcement trainers are challenged most by trying to change full adult males.

His activity level may be contributing to his attitude, as well. He doesn't have as much energy as a puppy, but now he has determination. He can do a marathon now where he used to barely make a 10K, to use runners' terms. Your stress levels may be contributing to his tension, too.