thanks very much for your reply.
A common cause of respiratory issues can be avian chlamydia though many bacteria can cause this and also fungal causes.
This can be prevented in budgies a few ways: diet is important - a lack of vitamin A makes the lung tissue weakened and more prone to infection. make sure you provide an adequate diet, fresh fruit and veg and a high quality multivitamin and mineral supplement in the water such as Avipro avian. A seed only diet isnt adequate for budgies and generally makes them become sick. Seeded diets can be dusty also affecting the respiratory system.
Regular cleaning of the cage to prevent fungal spore build up and to keep ammonia levels low will help - these are toxic to the respiratory system and can lead to respiratory disease if levels are high. Many bird owners will have an air purifier in the room their birds are kept in to help with air quality as they are very sensitive to air pollution in the room- if you get one of these make sure its an animal safe one.
Budgies respiratory systems are very sensitive so aerosols, candles, fumes etc. can damage the lungs easily if used in the same room
2) chronic malnutrition can cause these signs, apart form vitamin A vitamin d3 and calcium are important - this also can be kept at a good level through adequate diet and also water supplementation.
the sickness is something that might be related or come be separate - if it was a one off then maybe not too much of a concern though budgies can get infections also in their crops which causes them to regurgitate, if he accidently breathed in some of this it could have set off the infection.
i would recommend booking an appointment with a vet when you are back from your holiday who has experience in birds, they can assess his breathing and prescribe appropriate medications which will likely be antibiotics and anti inflammatories, they may also recommend nebulisation with F10 solution at a concentration if 1ml f10 to 250ml water x3 daily for 10 minutes at a time - this will need to be done in a small container. Due to the sickness they may wish to look at some of the contents of the crop under the microscope to make sure there isnt any signs of yeast infection here
i hope all this information helps