Night training often happens as a separate step, and often months (or even years) later than daytime success. Kids can't help this – the bladder and nervous system just need whatever time they take to mature. Poop training is also frequently a separate stage of training, because the signals and the effort required are different.
The usual wisdom is to wait until the child has had two weeks of dry nights to go for underwear at night. Too many 'failures' can be hard on a child's sense of maturity and competence, and it sure does make for a lot of sleep disturbance and laundry. I'd just work on the daytime success first, and let your son know it's just fine to sleep in diapers until his body is ready to stay dry at night. No amount of effort on his part will make a significant difference.
The evening walk is a nice idea. Some kids respond quickly to taking a drink, hearing running water, or splashing their fingers in a bowl of water. You might give those a try while he's sitting on the potty.
In case this is useful, acting over-the-top excited about every potty success can backfire emotionally. Your quiet confidence that he'll get it when he's ready will do a better job of helping him stay calm and focused on sensations than anxious anticipation of cheering (or whatever). The problem is that the opposite of all that celebration, when he doesn't have success, feels even more like failure. And failure eventually leads to reluctance to even try.
Good luck to you both. Sounds like he's getting closer!