I have family in the outer hebrides, grew up visiting in holidays for prolonged periods of time and personally, I couldn't live on an island. That includes the bigger islands like Lewis, Skye and Mull which have better infrastructure than the smaller islands.
The ferries are a nightmare at the moment. It can be really hard to get a booking, meaning that travel in emergencies can be problematic. We had a family member struggle to get off an island in the event of a death in the family.
The cost of housing and land may be cheaper compared to elsewhere in the UK but the cost of living is high. Food shopping is considerably more expensive, as is fuel and travel. You would really need to factor in regular trips to the mainland to stock up. Every time we have family over or we are travelling over, every nook and cranny of the car is packed with essentials that are expensive to buy on the island.
We stay in Oban and some of the children from Tiree choose to board at the school hostel in Secondary, rather than be educated on the island, as there are more courses available and the social opportunities are greater. It is worth factoring in, as it is alit of travelling for children, over and above their education.
Personally, I think your son would struggle to integrate on Tiree if he didn't attend school, as there aren't alot of extra curricular activities and then those that do run, the children would already know each other, making it harder to break in to established groups.
Medical care can be a nightmare. It's bad enough having to travel to a city for treatment from Oban, never mind the logistics of travelling from an island. We have only just managed to get an orthodontist, so up until last year, people were travelling between 100 and 200 miles for orthodontic treatment.
Also what people have been saying about the winter is true. It's long, cold, wet and dark. We once got stuck, as the ferries weren't sailing and it was grim.