Postcode into this website to check what type of aerial you need at a height of ten metres at your location.
http://www.wolfbane.com/cgi-bin/tvd.exe?
Also - take your TV into your son's bedroom and try it there with the aerial connection direct from the aerial. See if it will pick up the freeview channels there.
If it doesn't then the aerial signal is not good enough. You may need an amplified outdoor aerial. If it does pick up freeview then signal may be OK but weak. Try a TV booster the two outlet type. This in place of a splitter. One coax connection to your son's TV, other coax connection to the co-ax cable you have leading to your own room.
Even with a booster you may have too much loss on the co-ax extention. Don't use a longer cable than necessary. If still weak in your bedroom then try an additional booster (a one outlet type which gives the highest gain).
The above are only try it and see solutions. If results are still bad you may still need an amplified aerial feeding the two (or more) rooms.
http://www.maxview.co.uk/acatalog/TV...&_FILTERS.html
See the amplifier booster data and freeview problems on this website links :
http://www.satcure.co.uk/map.htm