Adding a joint owner to my credit card?

I have about 690 credit. My significant other is still a full time student with very little income and no credit history at all. I just got a new Discover credit card I am using for daily purchases and want to add him to it (we pay it off every month).

Will adding him as a joint owner build his credit?
Will it risk lowering my limit?
Will it negatively impact my score?

We want a joint credit card for practical reasons, but we'd also like to get his credit active. We're hoping to take out a mortgage in about 2-3 years. At that time his income will be higher than mine and we should have no debt.

We are not legally married yet for personal reasons, but we've been together a long time. We live as one household and already have our finances mixed, so cut the lectures, please, and give me straight answers.

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