Does masturbation (duration, intensity, frequency) affect actual intercourse? Yes, it certainly can. In your case it might not be an issue, but it can be - it depends on a number of factors.
If he has just gotten out of the shower after doing himself, and then you want to be intimate, he may have some problems. I presume he is in his mid-30's, like you, and while most men this age will be able to go a second round within a half hour, some can't and even if he can get an erection again, it might be more difficult to keep it, and more difficult to ejaculate. How difficult, again, will depend upon the man and perhaps how many ejaculations he has had in the previous days. Some men can do it twice a day every day since way back when, but others will run out of steam very quickly.
So, it is difficult to say exactly what is going on. I would tend - based on averages and with that assumption that he is around 35 - that occasional masturbation on his own should not be causing this. I do think he should get a physical exam from a doctor and mention this problem. Sexual difficulties can be the early signs of various diseases which, if caught early, can be cured.
As for better masturbating him, it could be the pressure applied, but sometimes a man needs a certain rhythm to ejaculate, and he may need to adjust this rhythm as he reaches the point of no return in a way you can't determine. Some other men, however, find the difference between his own technique and the female's technique to be sufficiently stimulating that he can reach orgasm more easily. I guess you need to ask him but, just as you can probably do yourself better than him, he can probably do himself better than you.
But, bottom line, I think he should consult a doctor for a full workup just to see if there are any physical factors at play. His difficulties - unless he is masturbating twice a day - are not normal for men in their 30's.
In closing, while you are very tolerant of his masturabatory actions, it is very possible they are interfering with your combined sex life. If he is masturbating alone due to an aperiodic sexual frequency to get him through the dry spells, and when you notice a problem is after he masturbated and you just happen to want sex later that day, then you may try to attempt to have intercourse on a more regular schedule. Knowing, say, you both decide you are going to make love on Fridays and Wednesdays, he will be less inclined to masturbate on those days, and probably the day before. I mean, if he is standing in the shower Wednesday morning, if he knows he will have sex that evening, he may decide not to masturbate, whereas if he thinks it won't be until Saturday he will get some, he may go on. And, yes, men are reluctant to speak with others about their masturbation, as oftentimes they are not thinking of their spouses while they do it, and to admit that is to open up the potential for an upset wife who will wonder who their husbands are lusting after.