There's probably no good way to do this.
Most people associate the term "work visa" with employer-sponsored visas. These visas generally require at least a bachelor's degree, so they would not apply here. You can read more about these visas at the USCIS page Temporary (Nonimmigrant) Workers. The E-1 and E-2 categories might work, as they are defined somewhat vaguely, but the requirements to invest "substantial capital" or to engage in "substantial trade" between the US and Canada seem unlikely to be met here.
Your partner's sister can sponsor him for immigration to the US under certain conditions; she must be living in the US, and she must be at least 21 years old. (An uncle or aunt cannot sponsor a nephew or niece for immigration to the US.) If she lives in the US, she could petition for him to immigrate after her next birthday. If the petition is approved, he will then have to wait for a visa to become available, because the number of applicants every year is higher than the number of visas available. The current wait time for a sibling from Canada (or from anywhere other than India, Mexico, or the Philippines) is around 13 years, so this does not seem like it will be very helpful in this case.
Another option would be to apply for immigration under the investor route, but that also seems unlikely unless your partner has access to some family wealth. The EB-5 visa provides for the immigration of people who can create at least 10 jobs by investing in a US business; the size of the investment must be at least $500,000 if the business is in a "targeted employment area," or $1,000,000 if it is elsewhere. Targeted employment areas are designated rural areas and designated areas with unemployment at least 150% of the national average.
A final option is the "diversity visa" lottery, more commonly known as the green card lottery. Eligibility is determined not by country of citizenship but by place of birth. People born in Canada are not eligible to participate, but if your partner was born elsewhere he might be eligible to enter.
Since none of these seems likely to help, at least in the near term, it may be that your best option will be for you to look into moving to Canada, or to set up the company so it can function with your partner in Canada and you in the US.
