The escalation procedures recommended by other posters make great sense, but I want to focus on the options that may improve the situation before you take a more hard-line approach, since once you do there is no going back and things may get worse before they get better.
What I might do: First, go to the manager and tell him/her that you are having a hard time getting some answers from the dev, because you are finding it somewhat challenging to interact. Do not point the finger and mention hostility (i.e. assign blame) - simply say that you are struggling a bit with hitting a stride in communicating with this person. In any case, tell the manager that your proposed approach is to give it another shot and try to have a brief informal meeting with the dev around the questions you need answered to do your job.
Ask the manager for suggestions on what to do IF this doesn't work out and you continue having a difficult time get answers required for time-sensitive tasks. (Document this meeting in a note for yourself, and then do exactly as the manager says if this happens.)
Finally, suggest to the manager (or ask if it would be OK) that you will document the questions and responses from the dev in a follow-up email after that meeting, and cc the manager. This way if/when the manager gets that summary they will know what it's about.
Managers like it when employees solve their own problems, and will appreciate that you are trying to work out the situation while at the same time making them aware of it, assuming they would want to be aware of this as your superior.
Then go to the Evil Genius. Ask if he has a second of free time, and if not, when would be a good time to stop by with a couple questions.
When you finally catch him to talk, explain in a calm and matter-of-fact way that being new to this job and to the subject matter you are doing your best to learn but recognize you have gaps in knowledge, and his input is very valuable to you.
As part of this conversation, make it very clear how your and his jobs are tied: even if his back-end stuff works perfectly but you are unable to do your job well, then the front-end will be broken. Make the point that no matter how well his stuff works, it won't matter because the user will continue to encounter errors and bugs. So you wanted to talk to him because you want to make sure that his quality work on the back end is reflected in the quality of the front end application, where it matters from the client/user perspective.
Tell him you are doing your best trying to learn but in some cases have questions that would be very difficult or nearly impossible to figure out without additional knowledge or answers about back end functionality. Therefore, you have a few specific questions (have a printed list) to discuss.
Also, try to create an "in" for future similar conversations, e.g. "It is possible I may hit roadblocks as I continue working through this, so was wondering if you would be OK if I occasionally bug you with questions, I wish I did not need to take your time but if I look everywhere and still can't find an answer, I may need to come to you. Would that be OK?"
Hopefully all this will set the stage for more positive and productive communication going forward. The whole prelude above should take 2 min of your and his time before you get down to business. Be clear, firm, and neutral/friendly in tone.
After the conversation, follow up by email documenting the work-related questions you had, any resolution/answers that he suggested, or if you were unable to get answers (again, do not make this sound personal - rather than "you did not answer my questions XYZ, say "I was not able to get answers to questions XYZ"). Copy the manager.
If the conversation got nowhere and only generated more hostility, go to the manager, explain the issue and ask for input on how to handle it. From this point, start a paper trail of questions and response (or non-response) from the dev, with cc to manager.
Do not appear phased if he responds with hostility. Simply thank him for his time and leave. Then, refer to suggestions in previous posts for escalation with manager, etc. Good luck!