Hm.. That's disappointing.
Isn't it supposed to be a very painful process to stop taking lyrium?
That he dies or goes mad would probably be a bit too far.. But just nothing? That's really lame
Well he's not exactly having a fun time... Not only did his templar and military training instill discipline, but he has a strong will on his own. This is a man that was tortured by mages and still retained his sanity. The mere fact of his decision to stop taking lyrium in the first place sets him apart from someone more weak-willed, like Samson.
Aside from the above, there is a more game-related reason for not having his lyrium addiction issues presented as even more severe: his necessary role in the game. He is one of your three advisors, and although we learn of his agreement with Cassandra to remove him from command if he becomes unable to function, the developers can't really have that happen to a core member of the Inquisition, however that might differ from similar real life circumstances. So, he either goes back on lyrium and becomes more stable because his addiction is satisfied again, or continues to not take it and stabilizes on his own (with a pep talk and encouragement from his boss, bro, gal-pal, or lover.)
I told him to keep taking it until the war was over.
During the final cutscene with Cullen before the final battle, he mentions the fact that he's still taking lyrium and how he feels like he's broken his promise to himself. And after the final battle, when you talk to him at the party in Skyhold, you can pick the option "I made you a promise" to bring up helping him quit. That was actually pretty sweet, because the Inquisitor said "It's a good time for a fresh start."
Hm, while it might be interesting to see that, I don't think I could make the final choice to pick that in the game. To an addict, falling off the wagon is a huge setback and he would have to start over from the beginning, but this time he already knows the horrendous suffering withdrawal will bring.
Writing that, I've just realized that part of his torture in Ferelden probably involved being denied lyrium. This seems the most logical. Why would they give a templar lyrium? The mages will want it for themselves, and they certainly didn't care enough about him to go find his philter in the barracks and allow him to use it. As it was, they probably gave him the barest amount of food to survive. This actually would have been an interesting line to throw out during one of the conversations!