What portion of the work will you be doing compared to your partner?
5% seems somewhat low if you will be doing a significant amount of work, but putting you on a vesting schedule is pretty standard practice.
That protects your partner in the event that A) Something happens to you, and you are unable to work, or B) you decide you don't want to be involved in the project anymore. Without a vesting schedule, your partner would be screwed, as you would be able to walk away with 5% equity in his company without actually contributing to it.
All startups I have worked with have used some form of vesting schedule. And in my current company, all founding members are on a vesting schedule to protect the company. It's just the smart thing to do.
There's an equity calculator floating around somewhere, I haven't found it on a quick search, but if you check on-startups, and look around on here you should be able to find it quickly. Use that to negotiate a better equity share for yourself. Make sure you have justification though, otherwise when the company goes into later funding rounds, the investors will ask why each person has the ownership share that they do.