Emphasise the Give Way lines (a visual cue) and that he MUST slow down as he approaches them to make sure that there is no traffic on the major road, which gets to go first. Explain that traffic on major roads has priority over traffic on minor roads. A junction is where two or more roads meet, and the minor road is always the one that is coming to an end, while the major road continues, so get him recognise this. (A T-junction is an obvious example.) While "buggi" has given you some good advice I do not recommend the following: "...get him [your son], on approach to junctions, to tell you what he's doing and why he's doing it." On the contrary, your son should be focusing on what he is doing (i.e. his cycling). By all means, get him to explain it afterwards, but not while he is doing it. Why not practice with him at a quiet junction?