gilrera DNA 50 over heating?

My Sons bike, Starting when cold is fine,starts with fast idle revs then ticks over as it should, carb mixture setting /idle set o k ,after a very short time of rideing as temp increases,revs on idel increase , water boils up over heats and cuts out and will not start till its has cooled down ,water pump can be seen pumping back to bottle, rad is clear and fine, hot water to carb heating is fine, before buying this bike it was reported that the head had been repaired Help if u can. thanks