There are a few options for your dog but given that he is such a large dog one may be better than the other.
The most cost effective option that will get your dog back to walking is unfortunately amputation. Now this can sound really traumatic and extreme but most dogs do really well and adapt beautifully. The cost of this surgery will be bless than another repair called a total hip replacement.
You also may consider getting an opinion from a boarded Veterinary surgeon to see which option they think will be the best for your dog.
Without being to examine your pup I can only theorize.
Here are some other options but will be increased in cost from the amputation but may be able to save the leg and is function and may require a specialist surgeon to perform the procedure.
1) Internal supportive techniques: A “surgical bandage” procedure can be done to keep the hip in place while fibrous healing of supportive soft tissues occurs. There are many different techniques but all of them are designed to provide stability long enough so that fibrous healing might occur.
2) Reconstruction of the joint capsule: This can be accomplished in some cases by suturing of the joint capsule, if it can be identified. It can also be reconstructed by placing screws in the dorsal acetabular rim and running suture from the screws to the remaining joint capsule or a hole in the femoral neck.
3) Reconstruction of the round ligament of the femoral head: This technique involves what has become known as “toggle pinning” and includes several variations. A suture of some sort is fastened at the base of the acetabulum and is passed through a drilled tunnel in the femoral neck that emerges at the point of origin of the round ligament on the femoral head.
4) Triple pelvic osteotomy (TPO): TPO was developed as a treatment option for canine hip dysplasia. It has been suggested as a means of stabilizing a chronically luxating hip.1,12
5) Salvage procedures: Total hip replacement (THR) or femoral head and neck ostectomy (FHO) are surgical options for chronically luxating hips.1 FHO or THR appears to be the only successful way of dealing with spontaneous, atraumatic hip luxations.4