Have now got ds1 (nearly 14) into routine where he showers most days, but he still regards it as optional. Fortunately he doesn't seem hormonal and doesn't get very smelly, but even so...
He's going to London today for just one night. When I suggested packing some shower gel and shampoo, he just rolled his eyes at me and said that really wasn't necessary for just 2 days, he wouldn't wash anyway, and he doesn't have room in his bag.
Mine are all very clean at home - showers every morning and then again after sport. But when they go away it is a different story. I don't think dd or ds have ever taken shower gel, or even a towel away on scout/guide camp for example (even when it has been a week long camp). In fact, I double they clean their teeth much when they are away (though that may change as they discover boys/girls ).
I picked up ds after two weeks away yesterday and he was distinctly pongy, as were all his clothes (but they didn't have the option of washing anything, so everyone smelt a bit).
I wouldn't worry about when he is away, but keep to the routine at home.
I have two teens. DS15 showers everyday for a least half an hour and sometimes twice if he has played sport. He even has a skin care routine morning and eve. He walks around the house with a face mask on daily.
DS17 only showers if he has to go to work or special occasions! He looks a bit greasy some days and tooth brushing is a bit hit and miss. Dont let it be a battle. I like to `nag` (remind) them about their teeth.
He has been known to sleep in a t-shirt, socks and boxers (that he has worn all day) and get up the next morning and throw on some jeans and go to college . Plus he sleeps with the dog so wakes up looking like an Ewok!!
Its his choice though - I do nag him about washing his face etc. I try not to sweat the small stuff...
My 13 year old is very hit and miss with hygiene. He will lay in the bath reading for hours but not actually wash. His hair looks manky and I have to remind him not only to wash it but to comb it afterwards otherwise he lets it dry like a rat's nest. One day I sent him up to get washed and dressed and he was back dressed in a flash. "Was there any soap involved?" I asked him. " There wasn't any water involved" was his answer.
DS15 was the same until he discovered girls. Now spends hours in the shower and sprays lynx all over.