i am of the camp that usually says... nicotine level is a preference of the person switching, and to suggest a nic level would be difficult. 1 can argue: He smoked light cigarettes, he needs lower mg nicotine juice!" and 1 could also argue: "But he wasn't a chain smoker, and he needs a juice that will satisfy his nic craving in a quick vape session, because he will want to vape like he smoked (infrequently)"
The nic and PG level also depends on the TYPE of tank he is using... He's using a clearomizer. They sorta NEED higher PG juice, unfortunately. Higher PG juice (more PG than VG) means a HARSHER throat hit, but a THINNER (easier to wick) liquid. Are we really gonna suggest this guy either try a different tank system, or suffer dry-burns with a higher VG juice in his current setup? UGH. This is why I truly HATE that vaping has moved in the direction that it has... Now we've got all different types of tanks, and it's become very difficult to explain to someone the differences between high airflow/high wicking/low-res/etc and low-air/less-wicking/higher-res. !!!
I suggest your husband try vaping what he has. in my experience, the throat DOES get used to vaping, over smoking. if he continues to smoke more than he vapes, he may NEVER become USED-TO the vapor. that's just my opinion.... & how vaping effects ME... I cough when I 1st switch, but i eventually become used-to vaping w/o any cough/throat-irritation. unfortunately, ur husband has to use a higher PG liquid in that tank (well, he probably should, to keep it wicking the juice to the heating coil fast enough, PG is thinner) but... I would recommend he try a lower PG (higher VG) e-liquid, and maybe 9mg nicotine? IDK! it's HARD to make such suggestions... Just PLEASE tell him to KEEP AT IT... and if you DO have a brick and mortar vape shop in your area... BRING HIM THERE, and have him try all different mg of nicotine, and maybe 3 different tiers/levels of VG/PG ratio e-liquid, okay? find-out what works best for him, with the current setup he's got, as-far as nicotine and PG/VG ratios go, okay? Good luck on your vaping journey!