This is certainly a tough situation to be in. Do you have a therapist that your child/children routinely sees, and that you trust and feel comfortable with? If not, I'd find one, as the first step. Toddlers can be bi-polar, but it's not very common. It's more likely that he's feeling stressed/anxious,and it's coming out in some scary statements/actions. My dauther, who's 8.5 now, has some sensory processing disorder, and sensory integration problems. She also had a severe lazy eye. Many of her behaviors when little (running out of her classroom, spinning in very stimulating environments, biting herself,) seemed like something on the Autism scale. When older and she'd talk about not having any hope, and not wanting to grow up,being "tired all the time", and her head being in a fog, we were worried about depression. Through work with her wonderful therapist, consults with OTA in Watertown, a neuropsych eval, and vision therapy, she's much happier, and we understand her a lot better. We now know that many of those behaviors were due to her trying to have some control over the stimulation she was receiving (either to increase it or decrease it). That she felt "stupid" about many things (which she's not-above ave. IQ) because she couldn't do what other kids could due to vision problems (e.g. she had no depth perception, so she couldn't catch + throw things, or kick a soccer ball), or couldn't tolerate school assembly because it was too loud, or couldn't sit unsupported because her muscle tone was so low that it was exhausting for her to sit on the floor. She'd be so worn out from trying to just deal with the environment and her body, that she had no energy left to tackle what was being asked of her in school-causing her her to scream, cry, run away. Now that we have a better understanding of what she's going through, and helped correct her vision, we can plan ahead for the stimulating environment; we can utilize exercises to help her calm down, or rev-up; employ techniques such as giving her gum to chew so she can sit still to do her homework;and we can explain all this to her teachers so they know, too, and can support her in her struggles to get through the day.
I don't have the answer to what your son may be experiencing, but I wouldn't just accept the bi-polar diagnosis. Best of luck. I know you've had some tough days and nights. Take the time to take care of yourself too, so you'll have what it takes to take care of them