If your father is going to use the concentrator mostly at home (which I assume is the case):

• I lean toward a stationary 5 LPM unit — that’s often enough for moderate oxygen needs and is quite stable.
• The Philips EverFlo would be my top recommendation if budget allows, because of its reliability and brand trust.
• If you need something more budget-friendly, the Yuwell 8F-5A or even the generic 5-L option can work — just make sure you confirm oxygen purity, and check warranty & service support.
• Keep a pulse oximeter to regularly check his blood oxygen (SpO₂), so you know if the concentrator is adequately supporting him.
• Plan for maintenance: clean filters, ensure good ventilation, and have a backup plan if electricity goes off.