First and foremost, I don't think people should feel entitled to pressure you about either subject, unless you've asked for their opinions. Even if you and the father were both still 16 years old (which sounds mathematically impossible), you have an adult situation and you must handle it as adults. People who wish to support you in your adult lives and responsibilities should begin by treating you as adults.
Since you have asked our opinions...
My strong instinct, if this were my 2-year-old child, is that he should be allowed/encouraged to call the man who's there to raise him "Daddy". Waiting until he's old enough to understand and letting him choose is likely based on two ideas, both of which are good-hearted, but illogical:
1 - Concern that, once he knows about his "real" father, he might feel disloyal and guilty for calling another man "Daddy"... You might feel guilty about it, since you had a relationship with his father; and you know his father would still be in his life, if he could. But to your son, it will feel like any other good adoption scenario:
* He should appreciate his father's role in giving him life.
* He will be relieved to know that the reason for his father's absence is NOT that he (your son) wasn't wanted, but because of something his father couldn't control - death.
* He may feel sad for his father, who has missed out on his life; or sad for himself, that he never got to know his father.
* But none of that will change the fact that the man who has been around all his life will be the one your son feels is "Daddy" (no matter what he calls him).
* He might feel temporary disappointment when he realizes "Daddy" isn't his biological father, but ultimately he'll feel a little more appreciative of him, and a little more loved, when he realizes "Daddy" is playing that role completely voluntarily.
2 - The belief that, if you let a child choose something for himself, you haven't influenced his choice... Parents teach their children what to call them, way before the kids are old enough to think about this for themselves. By consciously avoiding the term "Daddy", you won't remove yourself from the issue of what name is used, you will train your son not to call him "Daddy". Plus, by telling him (in the future) that you wanted to wait until he knew about his bio-Dad, before letting him decide whether to call his step-Dad "Daddy", you would necessarily introduce the idea that there might be something undesirable (disloyal) about it...an idea that might not occur to your son, if you didn't suggest it.
Regarding getting married:
If the hold-up is really what kind of wedding you can afford, then what's the hold-up? People have beautiful, touching, memorable weddings on the cheap every day.
DH and I used a wedding chapel that included everything - fresh flowers, music, photography, etc., etc. - in a package price. We had a charming dinner at an inn out in the country, rented cabins and stayed up all night with our family, closest friends and a local guy we hired to play guitar and sing. Oh, and I was nearly 6 months pregnant! I found a beautiful white, silk sundress from Pea in the Pod for ~$100 (nothing, compared to the cost of conventional wedding dresses). Nearly every guest took me aside in the next few weeks to say it was the best wedding they'd ever attended.
My brother and his wife rented a lake house for one night, had a friend ordained to do the ceremony right there in the yard and had friends and family members pitch in with food and drinks. We barbecued, had a bonfire and watched the kids swim, paddle-boat and chase each other with bubble wands. Everyone hated to leave and wants to go back, every year.
My uncle got married at sunrise in a little gazebo on a public beach and the reception was a nice breakfast, outside. It was one of the most beautiful, memorable weddings I've been to and it cost almost nothing.
I don't hear you saying you're opposed to marriage itself, nor that you're unsure about marrying this guy. In that case, why do you want to wait, if you have a kid on the way?
Having raised kids with someone I never married (we kept saying, "We're committed, we just want to wait until after the baby's born...after we've saved more money...after, after, after...") and now, raising kids with my husband; I do see a difference in our attitudes, when normal problems crop up. The unavoidable facts are:
* Especially when you have kids, there will be troubles, conflicts and hurt feelings that make you think - at least temporarily - "What a jerk! Why am I with this person?"
* It's legally, financially and socially easier to extricate yourself from an unmarried relationship; and that subliminal knowledge can affect the effort you put into resolving conflict.