As long as the person inviting is a PR or citizen who send the invitation letter is not very important. The mother would be most obvious person to invite the son. Both parents could be included on the invitation letter. What is most important is proof of the severity of the medical situation to show that why the som should be able to come to Canada to visit the mother, proof that he will return home due to his strong ties to his home country (spouse and dependent children, employment including approved leave from employer, property ownership, etc.) and proof of savings to support his travels. Given the reason for travel the finances are not as important as for tourism but proof of savings is still still important given the length of stay requested and given that it doesn’t sound that this is a palliative care situation.