Ok I'm getting really confused, because I am going through a recruiting agency for my job over there and they are the ones who told me that he only needs a passport.
Sorry for the confusion. I think everyone's been clear that, yes, the maximum time that a visitor can normally stay in a single visit is six months, and that US nationals can receive their visa at a UK port of entry. In practice, most visitors stay for considerably less than 6 months and when IND hear that somebody wants to go the limit in one shot they will worry about it.
Everyone's been clear that the IND are given discretionary powers - broad discretionary powers. They are in a different chain of command than those who issue work permits (although both report in to the Home Office, it's at the ministerial level), and they are not obliged to honour a WP if they think something is fishy.
So your husband would need to convince the IO that he is a bona-fide visitor: i.e., not intending to work, intending to leave at the end, etc . And he will need to clear the accommodation and maintenance hurdles. To this point, the amount you are going to show is $3,500 - slightly over $500 per month - which is wildly optimistic for London (unless your employer is providing accommodation, in which case it's borderline optimistic). And you are the sole bread-winner of the couple, so the possibility that your hubs might work is another thing for the IO to worry about. 2 - 3 months, that's OK, maybe even 4 months on $3,500 could work.
Now as mentioned above, you could get waved through and walk out laughing. Or they might decide your finances are weak and drill down a bit. No telling what can happen. So if there's a borderline case, the only way to reduce the IO's discretionary powers is to hold an entry clearance. When you have one of those, the IO can only drill down about variances in circumstances and a few other things. Refusals where the person holds a prior entry clearance are VERY rare. The entry clearance gets everything nailed down beforehand, all the questions are asked and all the research is completed before you set foot in the UK.
I'm not saying you MUST do it, but rather you have option of getting one if you think it's an advantage to do so. If you decide to get one, your husband would fill out the VAF1 form and take it to the British consulate. It costs £36 and they would put a full-page sticker in your hub's passport.
Again, sorry for the confusion, and I hope this explanation helps. I suggest reading this thread again because there's lots of good advice here...