Well, okay, it doesn't seem like a bright idea at first, but he has a point there.
He's invested $1.70 in the pot and as soon as you shove your $5, there is $6.72 in the pot (including the small blind). He needs $3.30 to call to win $6.72 (which is 2 to 1 or he needs to win a third of the time to make this call profitable). This is also known as pot odds
If he thinks that you're shoving light (as people do in 5NL), than in this case, he's going to be right a lot of the time and this is a correct call. If you're shoving a range of only TT+, AJs+ AQo+, then this is a losing call and with all the weird things that people do in 5NL, then it would be break even or maybe slightly better (that is, if bluffs are included in this range). You also shoved in the big blind, so it looks a bit more bluffy.
Either way, even if you're shoving aces or kings every single time then a suited one gapper is a great hand to take down these hands with.