I remarried, with two kids under 7, to a man with a 5-year-old. I had 200k savings; he did not, and I made 3x what he did. We have no kids together.
What worked for us was to make sure that the we set financial and life goals together, before we got married and bought a house together. We worked through these kinds of questions:
Are you both comfortable with "our" money versus his and hers money? It is critical to be on the same page when one partner vastly outearns the other.
Is he planning to treat your son, financially, in the same was as his future biological child? We treat our 3 combined kids equally - same college fund goals, same opportunities, inheritances will be equal, etc - even though 2 get hefty child support to pay for part and 3rd doesn't.
Can you agree on the same long-term financial goals? Ours were that we needed $X to retire, we planned to put $Y in each kid's college fund, we don't intend to leave the kids much of an inheritance, we wanted house paid off by the time my child support ended, and we would take a vacation up to $Z every year (I insisted on that one)
As long as you have the same long-term goals, you'll be in a good place to make the short and medium term decisions. We pushed retirement off a year so that H could quit his job and go back to school for 3 years. We pay a little extra on the house every month so it will be paid off when youngest kid graduates.
Once you and your fiance have agreed on your goals, then you'll be able to see if a bigger down payment is best...or not.
Also, get lots of life insurance, now, when you're both relatively young.
And good luck! My h and I have been married 8 years now. We are much happier than either of us was before, and youngest kid once happily told his therapist that he has 4 parents - the "main" parents and the "second" parents, who all love him the same.