I have no idea why your boyfriend's junk is stuck in the junkyard. He's just 25, in the prime of his penis. It's fairly rare for a guy that young to go from instant erection to shy willy. One study found that just 2 percent of men experienced erectile dysfunction before the age of 40. If it's a short-term thing, it could be simple: Maybe he just needs to get more sleep or have fewer drinks.
The causes of erectile dysfunction include aging, high blood pressure, diabetes mellitus, cigarette smoking, atherosclerosis (hardening of the arteries), depression, nerve or spinal cord damage, medication side effects, alcoholism or other substance (drug) abuse, pelvic surgery including radical prostatectomy, pelvic radiation, penile/perineal/pelvic trauma such as pelvic fracture, Peyronie's disease (a disorder that causes curvature of the penis and sometimes painful erections), and low testosterone levels.
Three days after Michael was found to have a dangerously blocked coronary artery, surgeons inserted a stent to prop the artery open. Now he is keen to get more men going to their doctor to be checked up. "When it comes to sex, people keep things to themselves. But this is an easy way to catch heart problems at an early stage and treat them before the worst happens."
Parasympathetic input allows erection by relaxation of trabecular smooth muscle and dilation of the helicine arteries of the penis. This leads to expansion of the lacunar spaces and entrapment of blood by compressing venules against the tunica albuginea, a process referred to as the corporal veno-occlusive mechanism. The tunica albuginea must have sufficient stiffness to compress the venules penetrating it so that venous outflow is blocked and sufficient tumescence and rigidity can occur.
The dorsal artery provides for engorgement of the glans during erection, whereas the bulbourethral artery supplies the bulb and the corpus spongiosum. The cavernous artery effects tumescence of the corpus cavernosum and thus is principally responsible for erection. The cavernous artery gives off many helicine arteries, which supply the trabecular erectile tissue and the sinusoids. These helicine arteries are contracted and tortuous in the flaccid state and become dilated and straight during erection. 
To a significant degree, the public, particularly older men, is conditioned to accept erectile dysfunction as a condition of progressive aging for which little can be done. In addition, there is considerable inaccurate public information regarding sexual function and dysfunction. Often, this is in the form of advertisements in which enticing promises are made, and patients then become even more demoralized when promised benefits fail to materialize. Accurate information on sexual function and the management of dysfunction must be provided to affected men and their partners. They also must be encouraged to seek professional help, and providers must be aware of the embarrassment and/or discouragement that may often be reasons why men with erectile dysfunction avoid seeking appropriate treatment.
Your ability to orgasm is not connected to the prostate gland, although a man who has had a radical prostatectomy will have a dry orgasm with no ejaculation. As long as you have normal skin sensation, you should be able to have an orgasm with the right sexual stimulation. This means that treating your ED will allow you to resume a normal, healthy sex life.
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Another potential new treatment consists of penile low-intensity shock wave lithotripsy. This consists of 1500 shocks twice a week for 3–6 weeks. The purpose is to stimulate neovascularisation to the corporal bodies with improvement in penile blood flow and endothelial function. The use of low-intensity shock wave lithotripsy may convert PDE5 inhibitor non-responders to responders.47
Exercise and lifestyle modifications may improve erectile function. Weight loss may help by decreasing inflammation, increasing testosterone, and improving self-esteem. Patients should be educated to increase activity, reduce weight, and stop smoking, as these efforts can improve or restore erectile function in men without comorbidities. Precise glycemic control in diabetic patients and pharmacologic treatment of hypertension may be important in preventing or reducing sexual dysfunction. 
Vacuum devices for ED, also called pumps, offer an alternative to medication. The penis is placed inside a cylinder. A pump draws air out of the cylinder, creating a partial vacuum around the penis. This causes it to fill with blood, leading to an erection. An elastic band worn around the base of the penis maintains the erection during intercourse.