There is a possibility of a bay. We don't know what the mare's base color is, only that she is gray and her dam was sorrel. That leaves a multitude of possibilities. Even if the mare was also a sorrel based gray, she can carry an agouti gene without anyone knowing because agouti doesn't act on red gened horses.
Just so you know, though, breeding to a paint mare, even if she is homozygous for color, doesn't guarantee a colored foal. I've got a horse out of a similar breeding, perch stud, paint momma, and this was the outcome
The only white on him is a striped face and he has horrible conformation, in spite of both sire and dam being well conformed.
To be perfectly honest, you're brother would likely have better luck getting something nice if he just went out and bought what he wanted. Horses, especially grade horses, are going for a pittance. Compare spending maybe $200 for a nicely green broke half draft to hundreds in vet cost just during the pregnancy, plus the risk to the mare, plus the feed and care for an addition 3+ years before the foal is even ready to be ridden. We're talking thousands of dollars and being afoot for at least 4 years when he could go pick out the color and temperament he wanted within just a few months and be riding immediately after purchase.