I have an 80-year-old uncle who suffers from heart disease, asthma and kidney disease. He asked me to perform his obligatory hajj on his behalf after he passes away. Is it permissible to perform hajj on his behalf while he is still alive?

If the case is as mentioned in the question, there is no objection if the inquirer performs hajj on behalf of his ill uncle under the following conditions:
1. His uncle is unable to perform hajj himself.
2. The inquirer is commissioned by his uncle to perform hajj on his behalf.
3. The inquirer has already performed his own obligatory hajj.
The above answers the question and Allah Almighty knows best.