For the purposes of immigration sponsorship: it makes absolutely no difference that your husband adopted your child. Your son is eligible to be sponsored by your husband for permanent residence because he is
your child (assuming he is under the age of 22 and not married or in a common-law relationship) and you are being sponsored as a spouse. He is included on the application as your accompanying dependent . . . the adoption has no bearing whatsoever on the sponsorship process except for, as Alice100 mentioned, the issue of you needing permission from the biological father for the child to immigrate. If your son's biological father gave up all claim to him (which
might be the result of the adoption), you should not need his permission for your son to immigrate. Otherwise, you'd need his signed, notarized consent.
Whether the adoption will be legally recognized in Canada (or not, as Alice100 implied) is another matter entirely and not applicable to the sponsorship process. Just because your husband adopted your son, don't confuse this
spousal/dependent child application process with the process required when someone sponsors an adopted child from another country. They're
not the same thing - and the requirements are VERY different . . . primarily in regards to medical inadmissibility.