Hi, u said the problem is the duration not erectile dysfunction,, if u slip him viagra into anything u might induce a heart attack and it will do nothing but give him a harder and painful erection.
So, u want something to keep him going, n im sure since u want him to enjoy it too u wont use local anasthetic. There is this group of meds, SSRI, selective serotonin reuptake inhibitors, one of their uses is premature ejaculation. You can try that, its relatively safe, Fluoxetine is maybe the safest of them.
But, if he is taking ANY meds dont even think about it, specially MAO inhibitors, its a lethal combo.
Good luck you two











