Hi C., sounds like you have gotten some good advice already. I am going through a similar situation with my daughter who is a little older - 15 mos. Her biological father (also short intimate relationship) who hasn't spent time with her in more than 9 months is now pushing for "Rule 22" visitation, Friday - Sunday. I am of the opinion that this kind of forced separation/bonding is not to anyone's advantage, except adults who are trying to prove a point to one another.
So! Due to the hostile nature of our relationship, "talking it out" has been a challenge - although mediation or counselling might work for some disputing parents, and the courts are now involved to the point that our daughter has been assigned a legal Guardian to look out for HER best interest, as opposed to either of ours.
For you, sweet Mama, I say better safe than sorry, and if your child is spending the majority of his time with you, then probably YOU know what is best for him, especially right now when he can't necessarily articulate himself. I think there is nothing wrong with taking it slow, and the less traumatizing the situation is for both the child and his father, the better their relationship may be in the long run. I think a phase-in is the way to go, so that everyone can see how things are going to work out, with baby steps.
Your son won't remember what's happening now, and right now HIS comfort is first priority, which may mean making sure YOU are okay too, since you are his primary care giver.
I agree with several of the other moms that have shared their great advice to go with your gut - only YOU can know what is best for YOUR situation. I encourage you - don't feel pressured by "the book" unless someone can justify why it is in the best interest of your child. They're just guidelines. You're the Mom, and you're the one who has taken responsibility for the love and nurturing of this little boy for the rest of his life - no matter who else he has a relationship with or not.
Best wishes and hang in there. Your concern shows your love!