Dyeing a doll white

I think if you search the forums for dezomibfication it will bring up a lot of threads about minimizing the yellow tones for a doll. You probably won't be able to get him back to a pristine white; resin will mellow/ yellow over time regardless, and you can't dye things white in general, only bleach them, so you won't find a white dye. I would advise NOT using the rit whitener. It's bleach and I don't think that's safe to use on resin.
I think most of the formulas for helping tone down the yellow use some kind of oxiclean. If you get him light enough, you could dye him with a light purple toned dye to neutralize the yellow, which might help tone down the yellow tones as well.

 

Dye is by nature a deposit of color. White in this sense is an absence of color, so you will not find a white dye. The alternative in this way is opaque white paint. Painting an entire doll will present a lot of unique problems long term.
There are tutorials on how to refresh resin color I think some call it de-zombify or de-yellow. Through different methods like sanding, adding pink dye to cancel overbearing yellow and using treatments like oxyclean to reduce oxidation yellowing.
But the most honest thing I can tell you is that it's important to accept that an older doll will literally never look exactly like it did when it was first manufactured, by nature of the materials. So the appearance could be changed and improved but try to be open about the results.
If you search here in the customization and maintenance I think there are detailed accounts of how to do some of these remedies.

 

If you do end up doing a light purple dye over the yellow (they're opposites on the color wheel, so they neutralise each other), I would suggest maybe 1.5 teaspoons to 3 gallons of water and a shorter dip, like 30-45 seconds to start. You could also add like half a teaspoon of pink to get it looking more like a skin tone, since that's what you do when mixing skin tone paint colors if one comes out too yellow.

 

@Arletstar have you tried yet to just scrub the whole doll with magic eraser first? Because of what you said and the doll is this old, it might be a big start just to do a vigorous clean. I had one very old one that started to get simply too yellow and gross looking this year and only cleaning it made a visible difference. It's a start! you'll want to clean it extremely well before doing anything else so maybe you can start there and see if there is any difference.

 

This ^^

I'm one of the folks who had a grass stained green doll, and a banana yellow doll, and restored them to a healthy NS shade. It's all about patience.
Lighten the doll with a good scrubbing, a paste of peroxide and baking soda 'soak' and then the dye bath of essentially a purple shade to restore the blues and reds (sometimes it takes two baths. A more blue tinted purple and then a lighter pinky bath). Worked well for me. The green doll (2005 BW from Luts...aka beauty green by 2008) has gone back to beauty green, but the NS doll I restored still looked better when I sold him nearly six years after his bath.

 

I'm curious if we can get a before pic and progress pics...for science...