So, before I say anything else: the border is closed to all non-essential travel through the end of this month at least. If this visit isn't essential he's not getting a visa. The rest of my answer assumes normal conditions:
We have a guide on exactly what to write:
https://www.immigroup.com/news/how-get-visa-invitation-letter-canada-5-steps
Whether or not you discuss money depends upon your uncle. If your uncle can show he has enough money to visit Canada then there's no need to mention you have the money to support him, though you can still do so to reassure the government. If, however, he can't show he can afford the trip, then you need to show that a) he will stay with you the whole time and b) you can and will provide for him while he's here. Unfortunately there is no publicly stated amount of money necessary, but it has to be more than what he plans on doing would cost. (For example, if he's staying hotels the entire trip, the amount of money he needs must be more than the cost of those hotels + the cost of other activities he's planning.)