The bad news is that you are working under unreasonable conditions right now. Not necessarily with malicious intendintent from your boss, but as long as you don't establish reasonable boundaries, he asks for more. The negotiation style of your boss might be OK when he deals with employees who perform grunt work but is inadequate with regard to the degree of autonomy and responsibility that comes with your new projects.
The good news is that you are in a very good position to negotiate better conditions. You just need to get more assertive.
In a first job, it can be acceptable and even very good to take on responsibilities above the official job description. He may officially promote you. Even if he does not want to change your title, it still allows you to explain to new employers what you actually did and be officially considered for better jobs. (Especially with startups this happens a lot.) I think that faster experience most of the time trumps the fact of being underpaid for the more important work. You should try and concentrate on the new projects instead of the support work.
At the very least though, you need to force your boss to respect your boundaries. If a deadline is not reasonable, say so. Or tell him that this is possible only if he gets someone else to do your initial support assignments and let him decide what is more important. Here you could subtly hint that this might even be possible without hiring new people. Whenever he overrules your objections, make sure there is a written trace of this.