You don’t need to pay a consultant or lawyer. It is very important that you add him before getting PR and landing. Just like your new baby, you need to add the 10 year old as well. I would include a short explanation that your husband only recently discovered that he had a 10 year old which is why you are only adding him now. I would assume that IRCC will ask for a DNA test. There is a procedure that must be followed for DNA tests for immigration and only certain labs are acceptable, etc. so don’t pay for a DNA test to include it with your application. If you want to do a DNA test to confirm paternity for personal reason you can certainly do it but may have to pay to do another one for IRCC. The son will have to do a medical. If the mother does not want the child to come to Canada adding him as a non-accompanying dependent is a good option. You will have to sponsor them before age 22 if they decide they want to come to Canada in the future.