Please refrain from to sowing a seed of insecurity in the mind if your boyfriend by hinting that you wish he was more well-endowed. If you browse through this site a little bit, you will quickly learn that men will obsess over penis size to a degree of compulsion. Just go to the penis enlargement section of the forum and you will see that some men are willing to go to rediculous extremes.
If he's comfortable and you're satisfied, then keep it at that. It might be possible to gain some minor increases in size, but, as someone else mentioned, it would take a significant amount of time and dedication to achieve insignificant increases in size (there are no scientific accounts that verify the effects of penis enlargement). And then there's all the crap you risk going through, if your boyfriend becomes insecure over size issues.
So use the time that he would have to stay at home, stretching his dick, for something more constructive. Go to a cafe and have coffee, travel to Peru and visit Machu Picchu, learn mandarin or Coptic language or something. Don't waste your time on penis stretching exercises...