Everything I listed is within a few blocks of Union Square. 5 minute walk at most to the Hilton (and the Hilton itself is actually a couple blocks west of Union Square proper). Hotel Nikko and Clift are a block from the Hilton. Kimpton has another property, the Hotel Monaco, over there as well.

If you open up to a 10-15 minute walk, you can also look at the hotels closer to the Moscone Center (Yerba Buena district). The nicer ones over there are the St. Regis, Intercontinental, and the other Westin. There is also the Four Seasons on Market. Really lots of choices - there are many cool/nice places to stay in downtown San Francisco. I have stayed with the larger more well known ones - and higher end of the price spectrum - but there are lots of other boutique places in close proximity that I am sure others will chime in with.

If he really likes to walk, you could look at the Nob Hill hotels (Fairmont, Intercontinental Mark Hopkins, Ritz-Carlton). That is actually my favorite area to stay at a tourist in SFO because of the views. But not as close to public transportation (unless you travel exclusively by cable car). And the walk back up the hill is REALLY a workout!

Given your original criteria, I still think the Taj is still a great choice. The Four Seasons may also be worth looking into.

One last wild card suggestion. One of my favorite hotels is also the Harbor Court Hotel, which is near the San Francisco Ferry Building. Would put you very close to AT&T park (so a walk to the game). But probably a 25-30 minute walk each way to the Hilton Union Square. I was lucky enough to have a client close to there once -- really loved the views of the Bay Bridge from my room.