All electrical devices produce radio waves, which can interfere with other devices. There are international standards for the production of electronic devices, which are meant to ensure that the likelihood of such interference is kept to an absolute minimum - but it seems that your son's DVD player may well not meet those standards.
If the interference is being picked up by your TV's aerial, you need one of these to filter it out:
http://www.maplin.co.uk/tvi-rf-filter-29605
(However my worry is that your neighbours might also need to use similar filters, as the interference from his DVD player may well be causing problems with their TV reception as well).
If (as is probably less likely) the interference is being carried through the main power supply in your house you'll need something like this:
http://www.maplin.co....inema-and-hi-fi-46830
Ideally though, the DVD player should be returned to the retailer (and a refund demanded) as it's not 'fit for purpose'.
Chris