There are a couple of different options you can try here. Though the reality is at 14 this should be his job to learn how to deal with the pressures that come with social activity's and responsibility. The fact is this is a very valuable lesson......and one he will use all his life if you allow him to deal with this.
I would suggest sitting down with him and asking him how he would like to handle the situation. Brainstorm with him and try to come up with a few different options. Here are a couple you can use:
Set a amount of text messages he can receive or send in each day. Now this can help, since he would then be required to speak with his own friends and decide who he wishes to text with and how often.
Set the times he can send or receive messages: This typically works out well, he will need to speak with his friends and request that they only text message at these times. This gives him the opportunity to use his own skills to deal with his responsibility's, his social activity's and his rules.
If his friends continue to break the rules set forth you can contact their parents and explain the situation or you can request that they not text your son. Most parents are pretty lenient about texting so you may run into a few parents who simply do not care. In this case you can block the number though his phone or contact your carrier and request a block.
At the end of the day kids will be kids and a large amount of texting is normal, keep in mind that children do not text like they do talk.....one text may consist of "Hi" and another one "Whats up" with each text being a question or comment.........so a 5 minute conversation could end up being 30-40 text messages between sending and receiving. The fact is that 42 texts in a hour is more like 1-2 5 minute conversations between teenagers.