Canaries do not wean until about 6 weeks so I am surprised your dad gave it to you early. They often look like they’re eating, but they’re ‘learning to eat’ and still need parental care. So, one thing you need to do is soak your seed or millet (not all - some of it) to make it easier for your baby to eat. Also look on this forum about making chop, you can use vegetables and chop them really fine. Consider adding boiled egg too. If you have pellet food, that can be soaked in water or apple juice.
Canaries only eat about 1/2 tsp total per day, so it would be easy to overdo the food. Offer each of the things I mentioned separately, and remove fresh food or soaked food after 2 hours to avoid bacteria or mold growth.
Do NOT ever force feed the baby. When they eat normally, the body closes off the airway ... when you put food in while they’re not begging the food can be aspirated and your baby could die.
I would not be in favor of the mashed banana, as canaries can get yeast and feeding banana could encourage yeast growth.
I’m waiting to hear how your baby is today. Just keep him warm by using a small light over the cage or a heating pad under the cage. If a bulb, I like a ceramic heat bulb and I put it on one end, so they can move away from it if they’re getting too warm.