My 17 yr old dachshund, Annie was hell on wheels until she was over 15 yrs old, she had an episode of IVDD when she was about 7, then again when she was 14 or 15. The first time she had severe neck pain (screaming/falling over when touched, lameness in her left front leg, and general reluctance to move), but she recovered completely after lengthy crate rest (6-8 weeks, don't remember exactly, but it felt like forever) and slow ease back into normal activity over 3-4 months. Because she was always a busy dog, the last few weeks of crate rest were difficult, and long term restrictions weren't really a feasible thing for us, so the goal was to get her back to her normal level of activity. I started giving her adequan about a month into her recovery, and I feel like it helped, as it had been really slow going to that point. Did some swimming as well, and walking in the tub with water deep enough she could barely touch. I think she was back to running/jumping around like a maniac after 6-8 months, but I continued to avoid encouraging things that I thought were likely to cause a problem, and also took a little bit more weight off her, to lessen further the stress on her neck.
The second time she had a problem she was either 14 or newly 15, and she never did get back to 100%, maybe 95, but she still had a little bit of an awkward gait at times. That time, she didn't have nearly as much pain, but she did have some CP deficits in her front feet, to the point that she would sometimes stumble when walking. I think she recovered more quickly that time, though not as completely as the first. She was already not doing as much jumping/high impact activity just due to age, so after about 2 months, I let her do whatever she wanted, and we resumed playing ball not too long after. She hasn't had any acute problems since, though she has considerably less muscle tone now and is sort of loose and wobbly everywhere- I do notice her front feet now occasionally cross over when she goes "fast", but don't know whether that's do to her neck of just general loss of perception.
I guess to summarize, I wouldn't have had any qualms about doing "preferred" level agility with her after her complete recovery from her first episode (she was doing higher demand activities in day to day life already), but it wouldn't have been feasible after the second. It'll just depend on how well he recovers, and whether you are willing to take the risk that comes with increased/higher impact activity. I would just take your time and see how he does, here's hoping for a full recovery!