Mine only feedsback if I (or something) covers the mic. Sometimes this happens if someone hugs me and puts their head against my ear, or if I put on a hat (I almost always use a scarf instead of a hat for that reason)
Also - it's not the frequency that causes the feedback, it's the amount of amplification. Frequency is like different keys on a piano - the lower notes are a lower frequency and as you go up the keyboard the frequency (pitch) gets higher. Amplification is the loudness of a sound.
I'd ask your audie if there might be a feedback suppression program that can be adjusted - or if your HA has a mic cover that might need replacing (the cover is like a tiny piece of foam/material that covered the mic on the HA, just like a "speaker grill cloth" covers the actual speaker for audio speakers (tv, radio, computer etc)
For the phone - it's important that he's not putting the phone right against his ear so that it's covering/crowding the mic.
If he has an ITE, CIC etc (non BTE) type - make sure he leaves a bit of "air space" between the phone handset and his ear the HA (NOT like a hearing person would put the phone directly in contact with their ear). If he has a BTE make sure he holds the phone handset close (but not touching) the MIC on the BTE which is slightly ABOVE "ear level". If he's not already doing so, he also should try using the T-Coil setting on the hearing aid as it is almost always MUCH better (espeically if he has moderate to profound hearing loss) than trying to use the phone in "Mic" position.
One thing that you can easily purchase at the audiologist's office is a special foam cushion that sticks on the phone handset that will also help minimize feedback.
Of course - it's also very important that the phone that you have is HA compatible! NOT all phones are and it makes a HUGE difference in clarity, understanding and feedback (HA compatible phones have a handset that is shaped in a way to prevent feedback issues when properly used).
If he's still having trouble with the phone - it might be worth looking into getting a neck loop that is compatible with his HAs and a (landline) phone that has an aux output (looks like a headphone jack on the side of the phone & is more common on cordless phones than corded ones)