The difficulty swallowing - especially if it was a sensation of his throat closing up - could be a symptom of a very serious allergic reaction. Chronic nausea has also been reported, possibly leading to vomiting. (Been there, done that.) It's not necessarily an indicator of nicotine OD, especially if the nausea starts very shortly after the first vape of the day.
Ok, he might have a problem with PG. People seem to fall into two broad classes on this one. Those people who can tolerate the amount of PG known to be in some flavorings, representing possibly up to 20% of a liquids total makeup. (That would be me. But I do take antihistamines every day due to other allergy issues.) And those unfortunate few who have a severe reaction to any PG in their liquids at all.
All he can do it try stuff with what is either called "high VG", or mistakenly 100 VG at some places - even though their flavorings may contain PG. If those don't work out well enough for him you have to head for the places that explicitly state everything in their liquids is 100 VG, right down to the flavorings.
If he likes 'baccy flavors the first place on my list for him to try is The Vapor Room. They have what they call a "100 VG" option. But this is one of the places with a "gotcha" attached to that. The only stuff they sell that is truly 100 VG, right down to the flavorings, is their tobacco stuff. And
only the tobaccos that don't have any extras in them - like Vanilla Tobacco, Hazelnut Tobacco etc.
Fortunately they have a lot of excellent tobacco flavors!
As a 100 VG user I love them all.
Also, at The Vapor Room site they have an FAQ posted up there about the ingredients. For their other flavors where there is PG in the flavoring base, the percentage of PG in their liquids is as low as only 5% - a lot less than most places where it's closer to 20%. If you have any specific questions you can e-mail them from the site for more info.
If you are considering The Vapor Room shout back here or send me a PM and I'll advise how to get the best deal on a first time customer order.