Jacinta,
My personal thoughts are that with low back pain, return to sport is not as predictable as with a tendon injury or ligament injury. The way his symptoms progress are far more important than a time frame.
let's keep in the mckenzie model for a moment if that is helping:
If his problem as truly a posterior derangment and Extension in lying helped initially, but is no longer helping, or his improvement has hit a plateau, you might need to progress the force, (Extension in lying with overpressure - either therapist generated or patient generated, or add mobilisations (Mckenzie mobs work a treat).
the phases of treatment for a derangment in Mckenzie is 1. reduce derangement and abolish symptoms, 2. maintain reduction, 3. restore function (i.e. flexion in his case) 4. prophylaxsis (continue with the reductive exercise as part of his normal warmup / cool down and once a day otherwise)
If however his symptoms no longer fit a Mckenzie Posterior derangment, he may well have some other issues lingering on - possibly more motor control issues (core stability / movement control) rather than mechanical stiffness or disc issues. this wil often happen as a result of fear-avoidance behaviour or pain inhibition of key muscles groups
But i think that if mckenzie helped initially, it's worth pursuing the extension principle to it's fullest including exploration of lateral glides (frontal plane work) if extension stops proving useful. - this is basic mckenzie principles that often get ignored or not taught - mckenzie is a lot more than the "Extension in lying" exercise. Sounds like you're using mckenzie already - let me know if you don't follow any of the above mckenzie stuff.
Keeping active is really important and even with a 'disc' injury, and i would think that a strengthening program can be continued with attention to his direction preference - keep him doing activities in Lx extension or neutral. jogging might be ok provided he doesn't slump and flex / sit in his cool down like so many do. Pilates (with an extension bias in his case) is a great form of exercise that would probably work in well with his physio.
discs like any soft tissue injury take at least 6 weeks to fully heal once they are unloaded and repetitive stresses are taken out of the equation. even after he feels symptom free (well before full healing takes place) he is at risk of reinjurying - so he should continue his reductive exercise even after he starts to feel 'pain free' and then continue on to work with you in regaining flexion in a safe and controlled way.
hope my thoughts help.
keep up the good work matey!
And lastly - don't put too much stock into CT's and scans - go with his symptoms. Broadbased disc bulges are really common and often do not correlate with symptoms. go with what you see and assess!