Sue,
The cracking noise he heard may well have been the ligaments stretching or tearing. If it is really bad a small fracture may have occured where the ligament attaches to the bone.
There is nearly always spectacular bruising in this area,(this is due to gravity & the many small veins in this area)and one of the most efficient modalities to aid in a quicker recovery from a severe sprain is Lymphatic Drainage massage.Check the net for a registered Dr Vodder School practitioner in your area.They have a very high standard of ethics and the training is internationally recognised.
Alternate ice and heat packs, applied regularly EVERY 2 hours also aids a quick recovery.
When you get your husband re Xrayed, be sure that the Xray is with medial, lateral and oblique shots as navicular breaks are often not seen with the standard medial and lateral views.
If you are really concerned & can afford it, a CT scan will show up any damage, but often a fracture doesn't show on a too recent injury as the "scab" hasn't started to form internally on the bone.
Even a severe sprain will often need a plaster cast or possibly a cam walker (very expensive, but easier to manage showers etc).
I did my ankle just 9 days ago and it was very severe, and I passed out from the pain several times. Also had the cracking on impact and again when removing my boots (Blundstones...)
Arnica 200c for 5 days morning and night with alternate days using lymph drainage on myself (very hard for a massage therapist to work on themselves with this injury) and an accredited lymph drainage massage therapist working for nearly 2 hours just 3 days after the injury, has substantially reduced the swelling and bruising.
I have also had magnetic field therapy directly on the ankle and chiropractic adjustments to knee, hip and spine. These have all helped to reduce the time I have to spend away from my clinic and I took my first client since the accident today. ( the doctor said I was likely to be off for about 6 weeks!)
I hope you will consider these other therapies as an adjunct to the spectacular healing powers of homeopathy.