As a 28 year old, I would never consider proposing after a year. Hell, I'm rounding 2 years in my current relationship and there isn't a chance I'll be proposing then either. He is rushing it, no matter the age. The bottom line is that if you don't feel right or ready, then don't go for it. Let him know ahead of time so he doesn't make a fool of himself either with some big scheme where you say no.
There are many things to consider...You're still very much in the honeymoon stage, you were only official 5 months ago, you haven't found the skeletons in each others closets yet. Yes, you're living together, that is great, but I'd ride that out for at LEAST another year, maybe more. There is no rush for marriage, a 28 year old guy isn't anywhere close to reaching the end of his shelf life, so he doesn't need to worry about locking down someone into marriage. There may also be a reason WHY he wants to rush it, but that is just speculation on my part. Ultimately, the reasons you each have aren't as relevant as how you feel. If you're not ready, you're not ready. Do not be bullied or fooled into changing your mind, life decisions need to be made with care.
"All is fair in love and war." - Francis Edward Smedley