Child with enlarged lymph nodes

Hello all, 

My first time posting so I'm sorry if I'm not doing it right!

I'll just get stuck right in and explain...

My son who is 3 years and 5 months has had an enlarged lymph node on the right side of his neck for around 4-6 months. It is not painful or hot or red, just like a marble under the skin. He's never had anything like it when unwell but at first I did think, 'Ah its winter, he might have a bug' and waited for it to go down... it only got bigger and I thought, 'I must get it checked out' but then lock down happened and 3 months went past in a blur. About 4 weeks ago I realised he had a second lump come up under the first. I called the GP who wanted him to have a face-to-face appointment where on inspection she found more enlarged/hard lymph nodes in his right armpit. Going through questions with GP - my son does have night sweats and he's always complaining of tummy and back ache (I'm pretty sure that's just a kid thing though!). He had no sign of ear or throat infection and temp was perfect. So she ordered some bloods to be done. We received these results last Wednesday at which she said his white blood count was a little low (4.5) and his lymphocyte count was 2.1 (apparently this is low?). She remarked that this showed lymphopenia? Also, no signs of infection or inflammation.

She did an immediate referal to paediatrics who wanted to see him the same day at the children's assessment unit. He had bloods taken for a blood film and chest x ray. The x ray was fine, bloods again showed absolutely no sign of infection or inflammation. They also found a 3rd one slightly enlarged in his neck but said the ones in his armpit are smaller than those in his neck. 
We went back today to the assessment unit who said his blood film came back as 'reactive'. She said this could be because of a virus etc but he's not been unwell as we have been isolating due to lock down so I'm not entirely convinced but I'm no doctor of course!
She told me to google this... but I don't have enough information from them to get an understanding as it either comes up with references to cancer or an infection which he doesn't have. He has to have repeat bloods in 4 weeks as the doctor wants to keep an eye on how they are.

I have been going out of my mind as to what the cause of this can be. Lymphoma was my initial thought because he's not been unwell, not even a touch of hay fever this year.  

I was laughed at by one of the paeds doctors for the lymph nodes not being 'big enough' to be worried about but they are growing slowing and more are becoming prominent. Im sure in another 4 weeks they'd have grown even more!

I don't really know what I'm asking but some insight may help me. Or anyone who has been through something similar, what should I be asking or looking for?

Thank you to anyone who takes the time to reply to me!

Love from,

A very worried Mum x