Diane,
First off I am not the Sue on TV - but I'm glad you learn a lot from her show.
The cream the doctor gave your boyfriend is for fungal infections. This is the kind of infection a lot of larger people get because the air can't get to their skin and it stays too moist.
SO this is what I suggest.
First your boyfriend should use a very hot wash cloth around his penis and let it stay there until it gets cool to the touch. It might sting a bit - and that's OK. Skin heals more quickly when blood flows to it. Heat draws blood to the area the heat is being applied.
Next he should use a hair dryer to dry that area. Hot or cool setting is OK. Just make sure the area is dry.
Then apply the cream.
Do this every night for 3 days. No sex for those 3 days. You are trying to get the skin to heal. It could look healed - but my guess is the rubbing of the skin during sex is causing pain to his skin.
(If his skin is really painful - he might want to wrap his penis in gauze bandage ( 1 layer only) after he puts the cream on.)
He should try to keep his skin really dry - except from the cream. He should apply the cream anytime he feels his skin starting to burn or pull.
He should try to wear as few clothes as possible for these three days too - and try to sit or lie in position as much as he can where that area gets air.
Before you have sex you need to go out and buy lubrication. You have to get a silicone based lubrication - not water based. Water based is designed to soak into the skin and you do not want that. You want silicone which lies on top of the skin and helps your bodies slide back and forth against each other with this this layer of silicone inbetween you. I use Eros and it is the best IMO. You can buy it off the links at the bottom of the page. Don't use KY - it is worse than nothing in your case.
If sex is painful - you are not going to have orgasms. It has nothing to do with his weight. It has to do with having painful skin. SO first concentrate on getting his skin really cleared up well. AND using a lubrication that will help.
If you use condoms - that might be making it worse. Be sure to buy condoms that are the right size - they should not be too snug around the top. If you are using lubricated condoms that could also be part of the problem - so switch brands and/or switch to unlubricated ones ( and use the new silicone lube you buy on the inside and the outside of the condom.
I suggest your boyfriend use this cream every day - even when his skin feels fine - for at least 3 months. His skin needs to get really cleared of this. But it is very easy to get back. So if it comes back bad ( and I am sure it will) do the heat, hairdryer procedure again. If he is lucky and it doesn't come back after 3 months- he can switch to using the cream once a week. As a larger person he has to figure this is a lifetime problem.
If the cream he is using is expensive, and it is not covered by a prescription, I know of a relatively inexpensive way to get the cream he needs. Just let me know if you are interested in this. ( post me your email address - I'll edit it out so no one but me will see it.)
If tis doesn't work - let me know - there are some other ideas you may want to try.
Good luck!
Susan