Help me make small dog and big dog get along?

I would take them to a pet trainer before that mastiff gets bigger and older. He will grow to almost full growth by 6 months. For now, never leave them alone together. The puppy is young right now but once he is full grown you will have a difficult time controlling it if you don’t learn to handle him now.
Till you can get a trainer. (try petsmart) Try holding him down on her down on her back whenever you see unwanted behavior. This is a submissive pose and she will at least get the idea that she is not the pack leader and you are. Hold her down till she is calm and hold her by the neck. This is behavior that other dogs do with each other to teach who is leader and boss of the pack.
You may want to do the same with the pom. Only make sure to do it when you see them doing something you dislike. Like aggressive behavior. They will get the idea in time it is something you do not desire or will tolerate. You must be consistant and don’t let any aggressive behavior pass because it is cute. With each other, this cute behavior can lead to their death.
Mastiff are usually very calm dogs but poms have a Napleaon complex. You may want to work a lot with the pom. If he is not fixed, you may want to get him fixed. This will calm him down a little.
But do get a proper instructor. They probably have several tips on handling the problem and can show you what you may be doing wrong that may make the problem worse than it need be.
Good luck. Hope it works out.

I agree with the all the advice offered by @Pandora. Please get advice from an experienced dog handler/trainer. Once you have a dog that is scared of another dog, it is quite difficult to reverse this, so proffessional advice is the way to move forward with this.

Also, something else to bear in mind:

DO NOT pet/stoke your Pom while he is distressed.

Petting or stroking your dog while he is in an anxious state, will just re-inforce the anxiety. He thinks you are petting him because what he is doing is ok, but it isn’t. (we normally reserve petting for when they are being good, when we are happy with them).
It’s difficult, but you should try and ignore this behaviour from him (while it’s happening), or try and re-direct his attention away from the anxiety.

Other, basic tips I can share with you are:

* Try not feed them together at the moment – especially from the same bowl.
* Give the older dog, time away from the puppy
* Do not leave them alone together (accidents can happen so avoid this for the time being)

Good for you for trying to sort this out sooner, rather than later.
Please do seek advice, your local vet and the internet will be able to help you source a good dog trainer (and maybe some puppy training).
Best of Luck!

The Pom is naturally and rightly very frightened of the Bull mastiff. Do not let them play together physically. Teach the mastiff to leave the pomeranian alone. She could, in her clumsiness, easily hurt your pom very badly.