I agree with windycanyon but would add that at 5 months I'd hope to be well into heeling. Every day you delay proper heel work whilst still walking the dog is simply reinforcing poor behaviour through repetition. Nothing to be gained and much to lose.
Dogs don't get much out of heeling, there isn't anything in it for them, and that's why some folks struggle with the concept; they have no way of rewarding the dog for continued good performance other than with verbal praise which given the length of the exercise loses it's effectiveness very rapidly. Saying "Good dog" for five minutes reduces itself to meaningless noise.
Using a prong collar won't teach the dog the one thing you want (the precise relationship between his head and your knee) it'll only indicate to him that in certain circumstances he'll be uncomfortable. Those circumstances will be defined by the dog, not you. You may tweak him for being out of position by your knee, but he may decide it's because you walked past a nice smelly gatepost. You'll get a result in time, but it will imprecise and slow.
Our group now teaches heel via clicker without even using a lead; quick, precise, and relatively easy given good observation and timing. When reinforcement is needed after the teaching phase a loose slip lead stopped up behind the ears is sufficient. There is a technique in using a lead in this way, it's not nearly as easy as it looks; again you need good timing, a reward system other than praise or treats and a degree of self discipline.
With a bit of luck come the Spring I'll have a pup of my own breeding; if I do I'll post a few clips of the procedure we use.
Eug