Call Inogen and explain issue to them and ask if they have a fix.
They seem to be great people but they got to the point of calling me about every 3-4 weeks to see if I was ready to buy an o2 machine from them. They were very cordial when calling but you may want to ask them not to call if u don't want them to.I did and they stopped but i would still buy their equipment.

First of all, you're right, his Inogen won't begin to give him enough O2 to keep him saturated, so that's out. No portables will produce at more than 3 lpm.
If he can get LOX, that's probably his best bet, and would give him the most mobility and the longest times away from home, depending on which of the dispensers he can get. Unfortunately, most providers don't offer LOX now. Your provider will need to let you know what they have and what they can get if they provide LOX. The downside of LOX is that it requires a regular re-fill and you have to be home for that, so it can tie you down one day a week. Goggle LOX dispensers to see what's available so you have some knowledge to work with.

Tanks can also work; they're heavier, a bit more awkward, and often don't last as long as LOX in the right dispenser. Goggle "O2 tank time at various settings" or something on that order, and you'll find a variety of tables that will tell you how long the tanks will last at various settings. Remember those are really best case scenarios. I use tanks when I travel and I always order at least one more than I know I'll use on the off chance of getting one that isn't completely filled. The small tanks will fit in a basket on a walker, wheelchair or scooter. You can fit wheelchairs and scooters with holders for bigger tanks, like an E tank.

@A MyCOPDTeam Member I can give you blueprints on how to build a custom electric wheelchair to adapt to electrical so you can use a home concentrator which is only one that can give you enough continuous 02 which is what you need,portable are pulse and mosst contiuous is 3lpm and doesnt last long,my email (Email address can only be seen by the question and answer creators)

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A MyCOPDT...

Sorry but what are oxygen capsules? Is it a pill or is what you call the oxygen Concentrator?

I have been using oxygen capsules for over a year now. Level of oxygen in my blood has gone up to 95%. My specialist says I don't require to use oxygen orally. Blessings and love xxx