When I wear pajama bottoms or like swim shorts without boxers I get a boner

It depends on your habit whether you will get hard-ons or not. As because you ar at 14-15, it is common to get exitement easily. So consult your parents and other close family members try to remain naked for some days at home which will help you to control such hard-ons. Thereafter you may use pajama bottoms etc.

Thoughtful mothers, when they begin to notice frequent erections in their boys, will begin a special nighttime ritual at least several times a week. At bedtime, the mom will give her son a very relaxing and thorough back rub which puts the boy in a very passive relaxed sleep, with many boys almost asleep at that point. The mom gently helps flip the boy into his back and proceeds to give a very soft and soothing massage to the boys neck and chest. This puts most boys into a quasi-sleep mode. At that point, the mom simply moves the massage lower on the boy’s body, including his boxer briefs. Many boys at this point aren’t mentally aware or register what’s going on. He won’t notice (or care) when her fingertips start to gently massage his boxer briefs. Won’t notice when his penis begins to lengthen and stiffen inside the briefs. But mom will. She will then begin to focus her massage on the material over the head of the boy’s penis, the most sensitive area of his young body. He will attain a full erection very rapidly. He won’t notice when her fingers then begin to massage the glans of his penis, below the head, nor that her other hand is gently massaging the fabric covering his testicles. At this point he is fully sexually aroused. The mom will soon feel the wetness through the fabric of his precum. At this point, the mom will stop and use her fingers to open the slit of the boxer briefs, allowing her son’s erection to naturally come out in the open air. Using her son’s own precum as a lubricant, she can gently massage his entire penis with her hand to bring him to a full and quick ejaculation in his sleep. Some mothers instead will simply lower their mouth onto the head of the penis and using a combination of sucking and licking the head, bring the boy to ejaculate directly into her mouth

If I was your father, and I saw you in public with that looked like a tent down there, I would say, "Congratulations on becoming a man, but you look like a boy in a man's body. That's unacceptable for any son of mine. Time for you to get some new underware."
Before puberty, you had a inch and a half long penis that was about 2' long when you had an erection. If what you wore down there was loose, it was no big deal. But your 6" penis that, in effect, turns into an 8" tent pole when hard is "too much information" for anyone around you who is paying attention. On your penis' journey from an inch and a half to 6", you need to find and learn to live with underware that will make your hard penis stay where it was when it was soft. It helps to match that up with loose outer pants to make what lesser bulge you do have in your underware less obvious. Living with a penis that wants to rip it's way through your underware, but can't, isn't very comfortable, but it is part of being a man.
The same is true of swim suits and their liners. If you think other guys aren't getting erections when they're are around "girls" in very revealing and sexy bikinis, think again.