[Closed] Help please

Evening all

First post on here, so apologise if this is in the wrong forum.

My son (now 3) has sever hypomobility and has a pucky balance bike which he loves and would always go on if we let him and if he was not in too much pain.

We have been investigating getting him a 'proper' bike but the ones that we could get him come with 'electric assist' and are upwards of £500.

Do any of you wise fellow members have any ideas how we could achieve the same effect without the huge price tag?

Many thanks

SA cyclist.