OK, here is my assessment.
Because of your short cycles, even if you had sex with your boyfriend no later than the 8th, there is a chance that he is the dad. Your first ultrasound does not suggest this, if you stopped having sex with him after the 8th. But your second ultrasound suggested it to some extent. (Unfortunately, for this purpose, the seventh-week ultrasound is usually more accurate than the 11th/12th week ultrasounds, which doctors always caution can be a week off if used to try to date a pregnancy.) Is the baby a girl or a boy? If a girl, this gives your boyfriend a stronger chance. (But a boy doesn't mean the opposite.) Your boyfriend's chances to be the dad are stronger if the cramping on the 11th and the 12th was ovulation, but it might not be.
But, you don't even know if the 8th was really the cut-off point with your boyfriend. If you continued to have sex with him after the 8th, and sex was always unprotected, then he has at least an even chance of being the dad, or better because of sheer volume.
Nothing rules out the second guy. For one thing, the second guy is exactly on the right day according to your first ultrasound. If the first ultrasound is dead right, and if you stopped sleeping with your boyfriend on the 8th, then the second guy is more likely (by a long way) to be the dad.
Obviously, if you do find out in the end that the baby is not from your boyfriend, you have to reveal the truth to all parties. The question you're asking is, how do you find out without having to reveal the truth to your boyfriend, so if he is the dad, you can just go on with your lives and let sleeping dogs lie?
I should say that this level of secrecy is not a useful thing for your stress, and keeping the secret from him will make things worse in the long run if you do find out later that the baby is not from him. But you could probably concoct a story where you tell him that you were so sure, that you didn't think it was necessary to open a can of worms, but just lately you have realized there is a need to do a DNA test, and you know it's only fair to him, etc. etc. If you tell it to him as though you only figured things out recently and you nobly want to give him the chance to find out, maybe you can squeak through the long delay in telling him the truth. I would only suggest going through such a performance if a test you have already done says he is not the dad.
This is not something that can be kept secret forever. It would be desperately unfair to everyone if the wrong guy took on the duties of fatherhood unknowing that he's not the dad, and besides, with how common DNA tests are nowadays for fun, your child is very likely to do one some day to learn about distant cousins, etc. As I already said, when a lot of time goes by on this secret, it gets worse and worse for it to be discovered. The last person you want to be furious with you for ruining their sense of who they are, is your child. So if you are toying with never telling, only never tell if you test and find your boyfriend is the dad.
Another couple of questions:
Does your boyfriend know there is a chance he might not be the dad? (Or are you keeping it a secret, and if so, no wonder it's 'killing' you.)
Does the other guy know there is at least a chance he is the father?
Where will you be if DNA testing does show the wrong guy is the dad? Does survival seem to hinge on this (such as, your boyfriend will leave you and you won't have any money, etc.) or are you OK in either case? Would the other guy step up in terms of support?
A lot of times, survival fears are what is behind ultra stress in not knowing who the dad is. If you have a good job and can take care of yourself in either case, you're way ahead of the game there.
Some questions:
When did you get the due date of June 8? Was it given to you at the first ultrasound when they told you that your pregnancy was in its 7th week?
The sex on September 13 was unprotected?
You really can't remember if you had sex with your boyfriend after September 8?
The problem with your seventh-week ultrasound information is that it isn't consistent with a person who has 25-26 day cycles. If they gave you the due date of June 8 then, and it was based on the crown-to-rump measurement of the baby, your cycle would have been behaving more like a 28-day cycle. Is it possible that you aren't always having a period every 25 days, and that it varies? Or did they not give you the due date at that first appointment?