Looking at what your adding you are effectively watering down the protein and fat element of the oxbow a bit. This is already on the low side (which is usually a good thing) so his diet is probably a bit low in both at the moment. Your choices are to stick to just oxbow (which I think is a bit boring) or to balance out the mix a bit with occasional wet meals, or by adding a bit more protein/fat to your mix. I would probably recommend adding in a wet meal a week going forward and for right now whilst he’s in a bit poor condition do it twice a week.
The kind of thing I mean by a wet meal is something a bit higher in protein than normal, so cooked rice with egg microwaved through, a tin of pilchards with a bit of millet, a tin of natural cat food and so on. The principle is to keep it balanced (so it can be fed to all rats) but top up specific minerals so along these lines. On the days you give a wet meal cut down on the oxbow or don’t feed it (or they will get fat)
70-75% cooked carbs: rice, cous cous, pasta etc – as they are young
10-15% high protein food; egg, fish, chicken, shell fish, cat food etc
15% veg
Add to this a few drops of good oil per rat, salmon/hemp/olive/omega/flax/linseed oil are all good choices, my fave being salmon oil for youngsters and linseed/flax for older rats.
You can add a pinch of a good general vitamin powder if you’d like, look for ones with added calcium, vit d and copper ideally. Not too high on the vitamin A.