You already know that the bread isn't fresh, and that bad-tasting fresh bread is irrelevant, anyhow. Personally, I would find another bakery or bake my own, since there isn't much chance of them listening to any business advice, because any decent baker knows rule #1 is the bread must be freshly baked that morning. If they lose enough business, it should be obvious enough to them what is causing the problem. Selling day-old bread is dishonest in my book (as a former bakery manager of 10 years experience). And I always try to avoid doing business with either dishonest or completely clueless businesses.
But if you must try to reform your baker for the sake of convenience (say you don't have another good bakery nearby, for example), this is what I would advise:
Calmly walk into your bakery and ask to speak to the manager or owner. Then introduce yourself as a regular customer of the bakery and very politely inform him that you buy bread very often, but only if it is of the highest quality and freshly baked the same morning. If he assures you that their bread meets your standards, reply:
I'm sorry but I must disagree, because the bread I've been buying lately doesn't taste freshly baked and wholesome. In fact, it is so unappetizing I'm afraid I might have to buy it somewhere else.
This should illicit at least one of about three different reactions:
He immediately becomes defensive and argumentative, rudely insisting that you don't know anything about bread, etc. You leave and find another bakery, because they're unwilling to improve themselves.
He admits the bread is day-old and he didn't see anything wrong with that until you brought it to his attention. He says no one else ever complains about it, so he didn't think it was a big deal. At this point he should be forthcoming about his future plans for the bakery:
a. He's going to immediately stop selling day-old bread or at least sell it as "day-old", at a discounted price. (In either case, since he was honest, give him a reasonable chance to improve his business practices). Or,
b. He's not changing his business practices to please anybody. "It's unfeasible," etc. Find another bakery.
He apologizes and explains that their policy has always been to serve freshly baked, wholesome bread. Offers you a sample and actually tastes it for himself. Now you can find out if you both share the same criteria for good bread:
If it is really bad but he thinks it is fine: find another bakery.
If it's really bad and he agrees with you, then his bakers need closer supervision, disciplinary actions, and replacements if all else fails. That is his job and he should be concerned. So, if you're confident that he will adequately solve the problem, then mentally give his shop a reasonable probationary period (not as an ultimatum for him, just a guideline for yourself).
Then thank him profusely for helping you with this problem, and let him know that you are looking forward to many more future visits to your favorite bakery. Also let him know that you would enjoy recommending their shop to your friends.
...But if the tasting sample turns out to be good this one time only, then they've got a problem with consistency. You should inform him that you only buy bread of reliably good quality. Again, that could be a serious personnel issue that must be properly managed. And again you have to decide whether or not to trust management to fix the problem quickly and correctly.
In many bakeries, free samples are always offered to customers so that they may judge the bread for themselves, before purchasing. Of course, the samples must represent the actual product sold. But any decent bakery would rather close than sell stale bread. So if that's what's happening, it's time to find another bakery.