What Are The Side Effects Of Steroids Used To Gain Weight?

Hi, 14 years ago my son suffered from ganglion neuroblastoma, this tumour was removed and was found to be contained within a membrane, the tumour was located within the adrenal gland which was also removed. Thankfully there was no further spread of the cancer. One year later he was also diagnosed with diabetes mellitus. I am very concerned because at the age of 21 he has been weight lifting with friends and has been offered steroids to help weight gain and improve his size, these tablets are dianabol 10mg, dosage up to 3 per day. My son is very keen to take these as he has always been small in stature and very thin. He is now old enough to make up his own mind, but if there is a medical reason on why he catagorically should not take them, I would be so very grateful if you would advise me so that I may show him. I am very worried that it will affect his health adversely. Thankyou so much for your help. worried mum.