As your dog is a mixture of Doberman Pincher and another breed. He is likely to inherit traits of both mother and father and hence he may not inherit all features of Doberman.
Height will grow naturally. There's no quick fix to increase the height. Your homework will be to provide him optimum diet and good mental and physical stimulation. Leave the rest on the nature.
Don't breed this dog further. You can get him spayed. It's not a dood idea to breed on pedigree of dog with another. When we do this, we spoil the breed.