That hardened, white substance seems exactly like something commonly referred to as "boar glue." Guinea pig males (boars), particularly young boars, can get a little over-excited when their hormones start raging, and, well, boar glue can be an uncommon result of that. "Boar glue" is a term for dried guinea pig semen, and it's been described as being "like superglue" when it dries. Once it's dry, it can be very difficult to remove. Often, owners are able to carefully cut the boar glue out by using blunt-tip grooming scissors, but care must be taken not to harm or accidentally cut the guinea pig's delicate skin. Soaking the area with a warm washcloth or gently massaging a little bit of coconut oil into the area may help soften things up. However, if cutting the fur isn't an option or isn't safe, simply waiting for the fur to grow out a bit more can make the situation easier to deal with, assuming that the glue isn't causing irritation or harm by being there. Vets should be able to assist you if the situation gets tricky or if you're too uncomfortable with handling him. (If the glue is blocking his urethra, see a vet immediately.)
Sometimes, boar glue takes the form of a "sperm rod," which is essentially boar glue that hardened inside his urethra. Usually, these fall out on their own, and they sometimes can be removed by unsheathing the penis and gently massaging or tugging anything visible out of the urethra. A washcloth or cotton ball soaked in warm water can help. However, if the rod is large, consistently stuck, or otherwise causing too much pain when trying to remove it, a vet should be seen, since they can assist in the process. If the sperm rod is making your guinea pig unable to pee, though, that can easily be a life-threatening situation that needs immediate vet attention.
As for his straining: is he making any noises of pain, such as high-pitched, loud shrieks, or is he just experiencing sensory-related discomfort from the glue? Guinea pigs can pee every 15-20 minutes, so if your boar's urethra is completely blocked (vet emergency), he would be in very visible and obvious pain. For poop, it doesn't hurt to check and see if any boar glue has gotten lodged in his perineal pocket (the "pocket" below his penis, between his testicles, and directly above his anus). Male guinea pigs do occasionally need to go through a periodic "boar cleaning" procedure, as their perineal sacs will accumulate grease, hay, fur, and sometimes feces (in cases of impaction) over time. Coconut oil can soften up the grease within the pocket, and a cotton swab (Q-Tip) can be used to gently scoop the contents out. If there's anything externally obstructing the anus, then it'll be at the bottom of that perineal pocket.
All in all, besides checking him out, cleaning him up when needed, and safely removing the boar glue if possible, just monitor him and keep an eye on him. If he displays any signs of serious illness or exhibits withdrawal behavior as a reaction to pain (such as being puffed up in a corner), then a vet visit is a must. If the boar glue stays there for a while, be sure to check the surrounding skin for infection/irritation, as well as cleaning any grease that may build up around or under the glue. When it comes to our beloved piggies, it's better to be safe than sorry! There's quite a few videos on YouTube on the topics of boar cleaning and boar glue to check out too, for better information. I hope all goes well with your little guy, and I wish you both the best!
