Death grip is a myth, at least in the physical sense. It could be argued that it exists in a psychological manner, but it's hardly the best term for something that's not physical.
Men who masturbate often (with or without porn) do so because they have a higher sex drive. Just like women. It does not make them unable to orgasm during sex with a partner.
It's most likely to just be performance anxiety in a new relationship (because decent men care about satisfying their partner - a man who never gets performance anxiety is likely only interested in his own orgasm).
If it doesn't improve as the relationship progresses, then there are some potential physical causes (none of which are to do with excessive wanking):
The penis loses sensitivity with age, this is part of the reason why young lads often suffer from premature ejaculation, but it's less common from middle age onwards. It's possible his sensitivity has dropped to the point that piv just isn't enough.
For men, most of the sensation during piv comes primarily from friction (whereas women feel more displacement). That's why condoms reduce sensitivity for men. So if you're using them, maybe try a different brand/thinner.
Equally, the wetter you get during sex, the less frictional sensation there will be. And just like pensies, vaginas come in different shapes and sizes.
It usually takes a combination of the above to make a man unable to orgasm during piv.
Funny that when a man can't reach orgasm during sex its his fault for being a porn addicted wanker, but when a woman can't orgasm during sex its also a man's fault for being rubbish in bed.