Normally vesting is over a much longer period, e.g. four years, so he is insanely trusting of you. For example, what if you worked for six months got the equity then just left? Then he might find it difficult to raise funding.

At such an early stage, 25% and no salary would usually be considered a co-founder. Of course, there's no legal definition so in actuality it's whatever you agree to. This situation is very common, where a founder didn't really derisk the company as much as he perceived.

For being a co-founder, the question becomes to what degree can you accelerate the business. Is funding holding him back and your background/involvement will encourage investment? Is the product what's holding him back, and you will 10x its dev speed? You'd make a logical pitch that with you as co-founder, the company will grow way faster than without. Additionally, I'd do some online equity calculators and present a logical case for a given percentage.

I'm also not sure what the strategy is with the seed range. You want to have enough to survive hopefully 2+ years AND propel the business forward. The low end implies maybe you'll pay yourselves minimally. However, you may need another raise very soon since it doesn't help blow up the company -- you'll have to hire/rent office space/marketing/etc.