I was going to suggest that Tay have audiology testing until I came to the part where you mention that his hearing is perfect. This means that his eardrums are moving normally in response to soundwaves and the grommets have drained all fluid that was previously present in his middle ears behind the eardrums.
If the audiology tests were done more than 6 months ago then these should be repeated.
Otherwise it is up to the speech therapist to advise you. She may want to get a second opinion or discuss Tay's speech problem with her colleagues.
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