dont try to over train him. go for the basics , heal ,sit ,stay etc especially as you say he has started to chase. Retrieving and unfortunately, chasing are natural instincts to the dog ,walking at heal etc is not. sort these first and the rest should follow but dont rush it . a few weeks or months without a retrieve wont harm. and definately dont show the dog a gun or shot game untill you are sure he is steady to your voice or whistle.