I have a son with long, thick, corkscrew curls that go way past his shoulders- I would hate to cut it for similar sentimental reasons.
You can get rid of the lice!
What worked for us is a combination of what everyone has suggested so far.
I do one treatment with Once and accept that I need a couple of bottles if it's bad.
Then every third day you need to do this until you break the cycle so for about a week/10 days.
You need vast amounts of cheap conditioner (I used to get it from the pound shop or Boots) - this is just to get the comb through the hair, don't worry about buying brand names, you need bottles of the stuff.
First of all, get his hair absolutely sopping wet. If he has low porosity hair, this is not as simple as it seems. But this is a really important step - the shower doesn't cut it for my son. It needs to be a bath and his hair needs to be submerged for enough time, repeatedly, until it is properly drenched. Then add the conditioner until it coats it all over the hair and scalp.
Get him to sit still with the iPad, or a film or games. I shamelessly use popcorn and treats as well to bribe my DS.
Also have a spray bottle handy and more conditioner.
Section the hair into small sections - add more conditioner (this is the key) and more water if it is starting to dry out. Comb through with a nit comb.
Rinse. Repeat every 3 days.
It does work, but the key is you have to get him to sit still and work fast so it's not too overwhelming.