Hi there,
My son is also 14 months old (dob 12/27/08) and he has pretty much outgrown his "one size" Happy Heiny diapers, so I would say stay away from that brand. He is on the last row of snaps lengthwise. He has always been big, like the 90th percentile for height and weight. Right now, he weighs around 30 lbs.
I don't know if you are open to diapers other than the "one size", but I love-love-love using simple prefolds with a Bummi's Super Whisper Wrap cover. My son is only on the size medium covers, and he has LOTS of room left in them. We have some size large covers, but he swims in them and we only use them in emergencies right now.
We don't do any fancy folding with the prefolds. We just fold them in half lengthwise, and slide them into the little pocket at the front of the Bummi's.
Bummi's are not the cheapest covers out there, but they will last through many babies! I bought almost all my Bummi's off of Craigslist, and I will be able to sell them again once my kids are done with them.
I would include a link to Bummi's, but every time I add a URL, my posting gets flagged for solicitation! (LOL) So just google it. Let me know if you need any help. Thanks.
Good luck!