As everybody who has more than one children knows, you can love several children at the same time. You love them differently, but equally hard.
So if your son's father recently spends less time with his son, the question to be asked is: Why? Why does he seem to be unable to love the child he has as much as he used to now that he is expecting another child? We know it is possible to love both, so there must be a reason for him loosening the contact to his son.
Is it too complicated for him to have his child around? (If so, why?) Is his new girlfriend against your son? (If so, why? I know from first-hand experience that children other than your own can enrich your own children's lives, rather than diminishing it.) What else could be between your son and his father? How can you (plural) find out and remove the obstacle? Is he someone you (singular) can talk to about it? Is she? Are you?
These are questions we cannot answer. Of course, you might not be able to answer them right away either. But you are in a much better position to find out than we are. It's very likely the best if you could manage to speak together about it. The three adults involved. Even if that hurts whomever of the three. If needed, search counseling, a moderator – whatever it takes. Because this is about raising a child. And almost nothing in the world is more important than this.
Let's not kid ourselves: The boy certainly will be better off with having both a mother and a father caring for him. And the experience of a smaller sibling, another household to learn (and influence) the rules of, more people to emotionally attach to, can enrich his life more than the mourning about the lost traditional family might hurt him.
If you cannot solve this, make sure you prepare to try to compensate for the loss of a parent later. This might require help, so be sure you know where you can get help, should such a problem arise suddenly. (Needing help, BTW, would not be a sign of an inability on your side, but on the importance of the role the father would be dodging.)
As an afterthought, a few thoughts to what you wrote in your question:
My son came home and told me that his dad now lives with a woman and that they are expecting a child together.
This can be very enriching for your son and for his new sibling. I have children from two different mothers, and the children dearly love each other and care for each other. And seeing this, what do you think can the two mothers feel towards the children which are not their own, but loved by her own? So in case it hurts you to see him have another family: Maybe it helps you to see that, for your son, this it could be very good.
Is there a way I can ensure that his dad continues to have full weekends with him. I feel that our son needs to spend time with his dad as well as me.
It is great to hear that you want your child to see his father regularly! And, yes, as all children he needs to feel loved by both his parents. Unfortunately, if the father truly doesn't want it, there is very little that you can do. So what remains is what I wrote above: Find out why this is happening, try to remove the obstacles, if possible. (Note: There are legal measures here in Germany and I heard of a case where a mother made the court order the father to spend time with his child under the watch of the authorities. Whether this helps the child can certainly be doubted, though.)