Could be a combination of "clippered coat", the change in the coat when a dog has been clipped several times, and the fall "molt". Even when clippered they still grow and shed undercoat, and fall and spring are the most heavy times.
It's also possible he has some sensitivity to wheat, which I assume is in the weetabix?, allergies will most definitely show in the skin and coat.
Here is what I would try:
Use a coat king, or a stripping knife, flea comb or the toothed side of the thinning shears and drag the coat to try and get as much undercoat out as you can. Then,
before you clip him, give him a bath in a good moisturising shampoo, or a hypoallergenic one.
Make sure he is thoroughly dry before clipping, you can bathe the day before you clip too.
Try an 8 1/2 blade instead of a 7, I find that alters the coat the least of all blades, and in fact I've had quite a few coats come back in shape after using the 8 1/2 for awhile. It's my blade of choice for all spaniels and setters

The 8 1/2 is only very slightly shorter than the 7 in the finished look, but I think much smoother.
If he's sensitive to fish or fish oil you can try some evening primrose oil.
It may take a few groomings and experiments to get him back in shape. Also be sure you clean the blades before oiling, with an old toothbrush and/or blade wash.