Op I was in a step-mum type situation (we lived together for 5.5 years and I basically acting in that role) and I would say slow down, try and emotionally detach yourself and look at it objectively. Even if he is the perfect man (which no-one is) step parenting is SO hard.
You might not feel the bond you expect to feel when you met the child and that will come as a shock. It can take a lot longer than you might expect.
When you do start to fall in love with the kids you then start to come up against different issues. You will be worried about their well being, if they are being fed well, if their mum is emotionally leaning on them too much, the choice of school, the hobbies they do / don't do and you will have almost NO control over any of it and it's very difficult to watch a child you are expected to love 'as your own' not be treated as well as you would your own. The only way to cope with this is to try and emotionally detach and tell yourself you can't care more than their parents but if you are a loving emotional person it feels very unnatural.
Then think about having your own kids. Will you want to take them to visit grandparents at Christmas? You might not be able to do that because of the schedule - I didn't spend a single Christmas with my family for the time I was with my ex. An awful lot of your 'family' time and holidays will likely end up more with your partners family rather than your own because they don't see much of his son. You might not get to take your children on holidays in the few years before they start school as you will need to take all the children and this will need to be in the school holidays so the price massively adds up.
Being a good parent means that the child will get prioritised over you but this does essentially mean you are just along for the ride and have to lump whatever decisions the parents make between them.
Then there the impact on you as a family making decisions. So you might not take a job that is a good career move as the commute will be too long and moving closer isn't an option because you need to stay local to his son. You'll be tied to certain areas / schools etc for your own children depending on where mum is living. If mum decides to move you might end up having to follow her.
It's easy to think 'I can love and care for another child part time' but it's not like taking a childcare job that you can leave it have time off. It can and will impact every choice you make for as long as you are together.
That's even before you start adding in the idea that he might not be perfect. Most common on step parenting board - nrp guilt, Disney dadding, expecting you to do the brunt of the childcare, favouring one child over another, placing financial burden on you. I'm sure many men don't intend to do this but it creeps in and social expectations of women make it feel justified to them. Read the the board to get an idea.
Finally - if you split you will have no rights to see this child again. I see my ex's kids once every couple of months which I am fortunate to do, but going from being someone that has taught them how to brush their teeth and helped with their homework to an occasional visitor is one of the most painful experiences of my life that I wouldn't wish on anyone.
My ex was very similar to yours in the beginning. I really believed he was perfect and he promised me all the things I wanted from life - marriage / kids etc. None of it ever happened. There was always a reason it needed to be pushed back or delayed, often to do with his kids. They will say what they need to say to get you on board.