If you hired or consulted with an attorney keep in mind they will tell you what you want to hear....business is business
As a MN provider I can tell you that the ONLY way I can legally deny a biological parent access to their child NO MATTER what the other parent says/wishes is if I have legal paperwork that specifically says I can not allow that person to have access to the child.
IF your son's biological father is listed on his birth certificate, I believe he DOES have rights as the child's LEGAL father. You DO NOT have sole custody UNLESS you have officially filed for it in the courts. Otherwise you are simply the parent that has physically raised your son but not having contact with your son or having any court ordered visitation does NOT automatically exclude the biological father from contact or from exercising his rights to pursue contact/visits etc.
IF the biological father chooses to give up his parental rights (which IME, isn't usually allowed....too many fathers doing so to avoid child support etc) then and only then can your son be adopted by your husband. I do believe in some cases the bio parent can give up rights IF there is another parent (your husband) wanting to take over. But normally, fathers can't just give up their rights as that usually leaves the state as acting "father" and having to financially support said child. kwim?
Anyways, from my understanding MN is NOT a state that favors mothers....as a matter of fact I read not too long ago that MN is one of 10 state's that heavily support fathers and their rights.
I have 2 DCK's right now where the family had similar situations as yours...one now has 50/50 custody (the 4 yr old DCK never had any contact with the father at all until then) and the other family, the father actually got custody with the mom having visitation rights and no, she isn't on drugs or unable to support herself or her other children...dad either had a better lawyer or really does have the means to offer the child a more stable home. The father has 70% primary physical custody and the mother has 30%.... up until court, the child lived 100% with the mother and only visited the father.
As for why your preschool was able to put your son's bio father on the do not pick up list I don't know...but if it came down to him showing up there with a birth certificate, the preschool would be in trouble for denying him his child. I know alot of daycare provider's that will say they won't let a child go but it doesn't mean they are doing so legally.