Help request: stuck fermentation--yeast energizer didn't help

I have a stuck fermentation problem. I'm making the Affligem Noel clone from Beer Captured. My OG was 1.090, FG was supposed to be about 1.018. I made a 1 qt starter of WLP 570 Belgian strong ale yeast and pitched 3/4 of it, getting very rapid start of fermentation. I saved a cup of the starter in the fridge for priming with extra yeast a few days before bottling (per the recipe). 3 days ago (2 weeks into fermentation) I checked the gravity and only found it to be 1.038 or so, and no evidence of ongoing fermentation.

I boiled up a combo of 2.5 tsp yeast energizer, 5 tsp yeast nutrient, 1/2 cup (about 3.5 oz) turbinado sugar, and 2 cups water, cooled it to 70 deg, and pitched it 3 days ago. I got a little boost in fermentation, bubbles in the airlock every 7 sec as the most brisk, but it didn't last long. 3 days later the gravity is now 1.035 with no further bubbling in the wort or airlock.

So the question is, what now? I'm at a loss as to why a strong ale yeast crapped out so soon, especially since I made a starter. I've never had this problem with Belgian high-gravity yeasts.

The options I see are:
1. Keep waiting and checking gravities for the next few days, maybe this is just REALLY slow fermentation
2. Toss in the saved remains of the Belgian strong ale yeast starter to try to re-boost fermentation, keeping in mind it's been 3 weeks since I first cracked the vial
3. Pick up a new vial of WLP 570 and pitch either the vial or a new starter of said yeast
4. Add champagne yeast to finish fermentation.

I'm slightly leaning towards option 4. Champagne yeast has helped me out several times with prior high gravity brewing issues. Any suggestions on these options, or other ideas? Thanks!

I'd give it another few weeks. Keep checking your gravity. Higher gravity beers take much longer to ferment. The yeast is probably tired and working slowly. If after a week, you haven't seen any drop in points, then you should start thinking about adding more yeast. Also, for ales over 1.070, I always do at least a 2000mL starter. Anything close to 1.100 and I double that.

Belgian yeasts are notoriously slow, but they also have very high alcohol tolerance. They don't flocculate well so they tend to stay in suspension longer. Don't be surprised if the fermentation takes 2 months. That's how long it took the tripel I made a couple years ago and that one had a comparable initial gravity. My advice is get another fermentor and brew a another beer in the meantime and just forget about your belgian beer. It'll be fine.

Also, the best way to deal with stuck fermentation is to not have it in the first place. If you pitch the correct amount of clean healthy yeast and ferment in a temperature controlled environment you should pretty much never have these issues.

Check out the pitching rate calculator at mrmalty.com

Mr Malty Pitching Rate Calculator

I think you massively under pitched the yeast. With 2 viles of yeast you would have at least needed a gallon starter for 1.090 wort. A one quart starter with only one vile of yeast is not enough. Really a one quart starter is probably not enough to increase the cell count by much at all.

I'd suggest waiting awhile like what another person suggested. If still having issues you can transfer the beer carefully to another fermenter and pitch more WLP570 but this time make a larger starter and pitch the starter at high krausen. That may or may not work. My experience with champagne yeast has only been able to drop the gravity by 3-5 points. Probably not enough for what you need.