If he was stamped in as a visitor on the 2-week Christmas visit, then the ILR is down the tubes anyway. I applied as a returning resident and was refused. I had been away for over 2 years, but the kicker was not being resident the last time I left the UK.
Check the stamp in the passport first, but from experience once the 2 year mark has passed, it's pretty impossible to get the returning resident visa.