I have a human kensai level 13 and am wanting to dual-class him to something else at some point what should that be, and at what level should I make the move?

Personally, I would recommend level 9 for dual-classing, as it takes only about 1 high level's worth of experience to regain your former abilities.

Since you are already level 13, that is probably the highest level at which it is wirth doing, so do the dual class now.

Mage is a popular choice, as the disadvantages of the two classes are redundant, but the advantages are not.

Now is the perfect moment for dual-classing, your character just gained the last extra half attack per round, waiting longer will cost levels and HLA picks in the final class and lengthen the dualing period where the kensai restrictions apply without the benefits.

Mage is the most powerful class to switch into with all the options the spells allow, you'll also be able to wear robes.

Thief is also a good choice, the "use any item" HLA thieves can pick later counters the kensai disadvantages.

Cleric or druid works too if you're low on priest spells, you won't be able to wear armor however.

Important is what role you need in your party and most important is how you want to play your character, even a pure kensai can be fun.

I'm fond of Thief, myself, since there aren't many characters that can keep growing as one.

Mage gives a lot of options, and you can write a bunch of scrolls to your spell book for experience if you want to level rapidly (and cheesily) to get your Kensai abilities back sooner.

I agree with Dtgreene - I'd have done it earlier, but now that you're at level 13 definitely don't leave it any longer.

Kensai/thief is very powerful - I had a lot of fun with it. And an advantage it has over dualling to a mage, particularly since you've left it quite late to do it, is that thieves gain experience more quickly than mages - it will take you almost twice as long to re-activate your kensai class if you dual to mage, and at this stage of the game that will make a big difference.

Just to back this up, the experience needs for level 14 are as follows:
Lv14 Thief: 880.000 exp
Lv14 Mage: 1.500.000 exp

You should've thought about that when rolling the character, because there are stat requirements for dual classing. If you're low on Int and Wis, thief is your only option (assuming you didn't gimp Dex).
Baldur's Gate: Dual and Multiclassing

Here is the most in-depth guide for Kensai/Mage dual-classing:

https://lilura1.blogspot.com/2017/09/Baldurs-Gate-2-Double-Class-Comparison-Fighter-Mage-Dual-Class-Berserker-and-Kensai-vs-Fighter-Illusionist-Mage-Multi-Class.html

Great article. :)

The article was too long so I broke it up into two. But then I expanded on the second:

https://lilura1.blogspot.com/2019/01/The-Rules-of-Dual-Classing-A-Step-by-Step-Guide-to-Dual-Classing.html

So there are two long articles now. Oh, well.