I am thinking about words, maybe used by older generations to approach a young man. And using some common word that also has some meaning along the lines of "young, handsome, lively, energetic or innocent boy". Which, basically, reminds the older generation of how they once were themselves. E.g. something like, "excuse me ...(young, handsome boy), could you carry that bag for me, please?"
Sir works. Coming from an old codger or an old lady, it throws them off their stride and gets their attention. Oddly enough, even millenials respond well to a little old-fashioned respect.
Here's my USA west coast point of view:
Use 'stud' if you really want 'handsome', but this could be seen as TOO complimentary.
Use 'kiddo', 'sport', 'junior', 'champ', and the like if you want to indicate they are young (and you are old).
Use 'buddy', 'man', or possibly 'dude' if you want a friendly, equalizing term.
A quirky word to call the person is Adonis. It may be a bit ambiguous so you could get away with saying it without sounding like you are hitting on the person.
▸ noun: (Greek mythology) a handsome youth loved by Aphrodite, the goddess of love
▸ noun: any handsome young man
Another term, stud (Hey there stud, can you go grab my bags) is heavy on the flirting side. I doubt a straight guy would say that to a straight guy. And as mentioned in comments it might as well have a phone number attached to it.
And then I will offer a third term, boss. Definition #7 pretty much sums it up. It could be said to a cool younger male without sounding like you want to have sex with him. This is heavy slang but understood by most.
You: Hey boss, what's up?
If it's man addressing a young boy in a (perhaps overly) familiar way, we have slugger, bud, buckaroo, champ, cowboy, or tiger.
Older women can use terms like hon (honey), bud or dear with little risk of it sounding like a come-on.
Added examples (Edited) "Be a good squire and help me carry these bags?" (suitable before the act) "You Sir, are a gentleman and a squire!". (exceeding expectations)
Addressing a young man as squire is complimentary, and was often used by older generations, usually after the fact. This is to say, if you were to instruct a young person as squire it would likely be taken as a condescending insult: E.g. "Squire, please carry my bags." would likely afford you a comment of "Who you callin' squire gov?" whereas after the fact it's a compliment of thanks. So something like "Ah you're a squire, many thanks." would be an entirely appropriate and complimentary form of thanks towards a young man - the connotation being: acting in accordance, worthy of a knight. Worthy of respect and a noble profession for a young man to pursue.
Historically, a young nobleman acting as an attendant to a knight before becoming a knight himself.
young man, whippersnapper, junior, lad, sonny
You could say something to the effect of "Hello handsome fella, would you mind helping me with my bag?" Or something friendly like that. This seems like a more polite way to address a young man while also being complimentary and respectful to him.
"Excuse me, kiddo, could you carry that bag for me, please?"
kiddo: (used as a form of address) a young person
Other options are champ, hero, looker, dreamboat, and adjectival gorgeous and handsome.
"Excuse me, gorgeous/handsome, could you carry that bag for me, please?"
That being said, the safest, simplest way to address a young person without sounding either false, condescending or whatever to them is by a plain, though polite: "Excuse me, could you carry that bag for me, please?".